Fixes #8782

It is expected that the proxy would be on a private IP address so the
configured proxy should be connected to regardless of the IP range
blacklist.

Debian package builds were failing for two reasons:

 1. Python versions prior to 3.7 throw exceptions when attempting to print
    Unicode characters under a "C" locale. (#9076)

 2. We depended on `dh-systemd` which no longer exists in Debian Bullseye, but
    is necessary in Ubuntu Xenial. (#9073)

Setting `LANG="C.UTF-8"` in the build environment fixes the first issue.
See also: https://bugs.python.org/issue19846



The second issue is a bit trickier. The dh-systemd package was merged into
debhelper version 9.20160709 and a transitional package left in its wake.

The transitional dh-systemd package was removed in Debian Bullseye.

However, Ubuntu Xenial ships an older debhelper, and still needs dh-systemd.

Thus, builds were failing on Bullseye since we depended on a package which had
ceased existing, but we couldn't remove it from the debian/control file and our
build scripts because we still needed it for Ubuntu Xenial.

We can fix the debian/control issue by listing dh-systemd as an alternative to
the newer versions of debhelper. Since dh-systemd declares that it depends on
debhelper, Ubuntu Xenial will select its older dh-systemd which will in turn
pull in its older debhelper, resulting in no change from the status quo. All
other supported releases will satisfy the debhelper dependency constraint and
skip the dh-systemd alternative.

Build scripts were fixed by unconditionally attempting to install dh-systemd on
all releases and suppressing failures.

Once we drop support for Ubuntu Xenial, we can revert most of this commit and
rely on the version constraint on debhelper in debian/control.

Fixes #9076
Fixes #9073

Signed-off-by: Dan Callahan <danc@element.io>

The last stream token is always known and we do not need to handle none.

The `RoomDirectoryFederationTests` tests were not being run unless explicitly called as an `__init__.py` file was not present in `tests/federation/transport/`. Thus the folder was not a python module, and `trial` did not look inside for any test cases to run. This was found while working on #6739.

This PR adds a `__init__.py` and also fixes the test in a couple ways:

- Switch to subclassing `unittest.FederatingHomeserverTestCase` instead, which sets up federation endpoints for us.
- Supply a `federation_auth_origin` to `make_request` in order to more act like the request is coming from another server, instead of just an unauthenicated client requesting a federation endpoint.

I found that the second point makes no difference to the test passing, but felt like the right thing to do if we're testing over federation.

This makes the "messages" key in the content required. This is currently
optional in the spec, but that seems to be an error.

Removes the trailing slash with causes issues with matrix.to/Element.

Adds the redacts endpoint to workers that have the client listener.

This table has been unused since Synapse v1.17.0.

Tests were broken due to an API changing. The code used in Synapse
proper should be compatible with both versions already.

This makes the CAS handler look more like the SAML/OIDC handlers:

* Render errors to users instead of throwing JSON errors.
* Internal reorganization.

Adds a new setting `email.invite_client_location` which, if defined, is
passed to the identity server during invites.

This adds an admin API that allows a server admin to get power in a room if a local user has power in a room. Will also invite the user if they're not in the room and its a private room. Can specify another user (rather than the admin user) to be granted power.

Co-authored-by: Matthew Hodgson <matthew@matrix.org>

This had two effects 1) it'd give the wrong answer and b) would iterate
*all* power levels in the auth chain of each event. The latter of which
can be *very* expensive for certain types of IRC bridge rooms that have
large numbers of power level changes.

The final part (for now) of my work to implement a username picker in synapse itself. The idea is that we allow
`UsernameMappingProvider`s to return `localpart=None`, in which case, rather than redirecting the browser
back to the client, we redirect to a username-picker resource, which allows the user to enter a username.
We *then* complete the SSO flow (including doing the client permission checks).

The static resources for the username picker itself (in 
https://github.com/matrix-org/synapse/tree/rav/username_picker/synapse/res/username_picker)
are essentially lifted wholesale from
https://github.com/matrix-org/matrix-synapse-saml-mozilla/tree/master/matrix_synapse_saml_mozilla/res. 
As the comment says, we might want to think about making them customisable, but that can be a follow-up. 

Fixes #8876.

This fixes an KeyError exception, after this PR the content
is just considered unknown.

If we see stale extremities while persisting events, and notice that
they don't change the result of state resolution, we drop them.

... and disable coverage tracking for mypy and friends.

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

* Use the simple dictionary in fts for the user directory

* Clarify naming