Neilj/fix threepid auth check

Clarify docs for public_baseurl

This is leading to problems with people upgrading to clients that
support MSC1730 because people have this misconfigured, so try
to make the docs completely unambiguous.

The problem here is that we have cut-and-pasted an impl from Twisted, and then
failed to maintain it. It was fixed in Twisted in
https://github.com/twisted/twisted/pull/1047/files; let's do the same here.

Add metric for user dir current event stream position

Move SRV magic into an Agent-like thing

In the debian package, make the virtualenv symlink python to /usr/bin/python3.X
rather than /usr/bin/python3. Also make sure we depend on the right python3.x
package.

This might help a bit with subtle failures when people install a package from
the wrong distro (https://github.com/matrix-org/synapse/issues/4431).

* Remove unnecessary ORDER BY clause

* Add logging

* Newsfile

this makes it easier to stub things out for tests.

it is only ever a bytes now, so let's enforce that.

this thing is now redundant.

... instead of the matrix_federation_endpoint

Pull the magic that is currently in matrix_federation_endpoint and friends into
an agent-like thing

Fix bug when removing duplicate rows from user_ips

Config option to disable requesting MSISDN on registration

This was caused by accidentally overwritting a `last_seen` variable
in a for loop, causing the wrong value to be written to the progress
table. The result of which was that we didn't scan sections of the table
when searching for duplicates, and so some duplicates did not get
deleted.

All this magic is redundant.

* remove dh_params and set better cipher string

Implement server side of MSC1794 - Federation v2 Invite API

Add groundwork for new versions of federation APIs