Port the PresenceRouter module interface to the new generic interface introduced in v1.37.0

Signed-off-by: Dirk Klimpel <dirk@klimpel.org>

This adds a new API GET /_synapse/admin/v1/username_available?username=foo to check if a username is available. It is the counterpart to https://matrix.org/docs/spec/client_server/r0.6.0#get-matrix-client-r0-register-available, except that it works even if registration is disabled.

Signed-off-by: Marcus Hoffmann <bubu@bubu1.eu>
Signed-off-by: Dirk Klimpel <dirk@klimpel.org>

Signed-off-by: Drew Short <warrick@sothr.com>

This is mostly useful in case the server offers TLS, but doesn't present a valid certificate.

Signed-off-by: Dirk Klimpel <dirk@klimpel.org>

Per issue #9812 using `url_preview_ip_range_blacklist` with a proxy via `HTTPS_PROXY` or `HTTP_PROXY` environment variables has some inconsistent bahavior than mentioned. This PR changes the following:

- Changes the Sample Config file to include a note mentioning that `url_preview_ip_range_blacklist` and `ip_range_blacklist` is ignored when using a proxy
- Changes some logic in synapse/config/repository.py to send a warning when both `*ip_range_blacklist` configs and a proxy environment variable are set and but no longer throws an error.

Signed-off-by: Kento Okamoto <kentokamoto@protonmail.com>

Add developer documentation to explain room DAG concepts like `outliers` and `state_groups` (#10464)

Setting the value will help PostgreSQL free up memory by recycling
the connections in the connection pool.

Signed-off-by: Toni Spets <toni.spets@iki.fi>

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

Signed-off-by: Dirk Klimpel <dirk@klimpel.org>

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

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.

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.

By referring to awaitables instead of deferreds.

Fixes #9602

Signed-off-by: Dirk Klimpel <dirk@klimpel.org>

* Fix a broken link in the admin api docs

* Rename 10321.doc to 10321.docs

* Rename 10321.docs to 10322.doc

Signed-off-by: Dirk Klimpel <dirk@klimpel.org>

Fix link in modules doc to point at instructions on registering a callback instead of ones on registering a web resource.

Related to #10251

The presence router docs include some sample homeserver config. At some point we changed the name of the [config option](https://github.com/matrix-org/synapse/blob/859dc05b3692a3672c1a0db8deaaa9274b6aa6f5/docs/sample_config.yaml#L104-L113), but forgot to update the docs.

I've also added `presence.enabled: true` to the example, as that's the new way to enable presence (the `presence_enabled` option has been deprecated).

This PR:

* Converts UPGRADE.rst to markdown and moves the contents into the `docs/` directory.
* Updates the contents of UPGRADE.rst to point to the website instead.
* Updates links around the codebase that point to UPGRADE.rst.

`pandoc` + some manual editing was used to convert from RST to md.