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.

This is to help with performance, where trying to connect to thousands
of hosts at once can consume a lot of CPU (due to TLS etc).

Co-authored-by: Brendan Abolivier <babolivier@matrix.org>

By referring to awaitables instead of deferreds.

This is part of fixing #6116: we want to put RC debs into a different place than release debs, so reprepro has to be able to tell them apart.

A few things here:

* Build the debs for single distro for each PR, so that we can see if it breaks. Do the same for develop. Building all the debs ties up the GHA workers for ages.
* Stop building the debs for release branches. Again, it takes ages, and I don't think anyone is actually going to stop and look at them. We'll know they are working when we make an RC.
* Change the configs so that if we manually cancel a workflow, it actually does something.

Fix messages from multiple senders in historical chunk. This also means that an app service does not need to define `?user_id` when using this endpoint.

Follow-up to https://github.com/matrix-org/synapse/pull/9247

Part of MSC2716: https://github.com/matrix-org/matrix-doc/pull/2716

Previously only world-readable rooms were shown. This means that
rooms which are public, knockable, or invite-only with a pending invitation,
are included in a space summary. It also applies the same logic to
the experimental room version from MSC3083 -- if a user has access
to the proper allowed rooms then it is shown in the spaces summary.

This change is made per MSC3173 allowing stripped state of a room to
be shown to any potential room joiner.