Fix a circular import, which was causing exceptions on boot if SAML
was configured.

Signed-off-by: Olivier Wilkinson (reivilibre) <olivier@librepush.net>

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

Synapse 1.38.1 (2021-07-22)
===========================

Bugfixes
--------

- Always include `device_one_time_keys_count` key in `/sync` response to work around a bug in Element Android that broke encryption for new devices. ([\#10457](https://github.com/matrix-org/synapse/issues/10457))

As per comment

Fixes https://github.com/matrix-org/synapse/issues/10456
See also https://github.com/vector-im/element-android/issues/3725

it's flaky on circleCI, and having to manage multiple CI providers is painful.

Port the third-party event rules interface to the generic module interface introduced in v1.37.0

Follow-up to #10332.

Add a new version of the R30 phone-home metric, which removes a false impression of retention given by the old R30 metric (#10332)

Signed-off-by: Olivier Wilkinson (reivilibre) <olivier@librepush.net>

This PR is tantamount to running 
```
pyupgrade --py36-plus --keep-percent-format `find synapse/ -type f -name "*.py"`
```

Part of #9744

It looks like it was first used and introduced in https://github.com/matrix-org/synapse/commit/5130d80d79fe1f95ce03b8f1cfd4fbf0a32f5ac8#diff-8a4a36a7728107b2ccaff2cb405dbab229a1100fe50653a63d1aa9ac10ae45e8R305 but the 

But the usage was removed in https://github.com/matrix-org/synapse/commit/4c6a31cd6efa25be4c9f1b357e8f92065fac63eb#diff-8a4a36a7728107b2ccaff2cb405dbab229a1100fe50653a63d1aa9ac10ae45e8

This adds an API for third-party plugin modules to implement account validity, so they can provide this feature instead of Synapse. The module implementing the current behaviour for this feature can be found at https://github.com/matrix-org/synapse-email-account-validity.

To allow for a smooth transition between the current feature and the new module, hooks have been added to the existing account validity endpoints to allow their behaviours to be overridden by a module.

These signatures would end up invalid since the joining/leaving/knocking
server would modify the response before calling send_{join,leave,knock}.

The postgres statistics collector sometimes massively underestimates the
number of distinct state groups are in the `state_groups_state`, which
can cause postgres to use table scans for queries for multiple state
groups.

We fix this by manually setting `n_distinct` on the column.

Signed-off-by: Olivier Wilkinson (reivilibre) <olivier@librepush.net>

Our documentation has a history of using a document's name as a way to link to it, such as "See [workers.md]() for details". This makes sense when you're traversing a directory of files, but less sense when the files are abstracted away - as they are on the documentation website.

This PR changes the links to various documentation pages to something that fits better into the surrounding sentence, as you would when making any hyperlink on the web.