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

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).

* Fix broken links in documentation

* newsfile

Use a database table to hold the users that should have full presence sent to them, instead of something in-memory (#9823)

At the moment, if you'd like to share presence between local or remote users, those users must be sharing a room together. This isn't always the most convenient or useful situation though.

This PR adds a module to Synapse that will allow deployments to set up extra logic on where presence updates should be routed. The module must implement two methods, `get_users_for_states` and `get_interested_users`. These methods are given presence updates or user IDs and must return information that Synapse will use to grant passing presence updates around.

A method is additionally added to `ModuleApi` which allows triggering a set of users to receive the current, online presence information for all users they are considered interested in. This is the equivalent of that user receiving presence information during an initial sync. 

The goal of this module is to be fairly generic and useful for a variety of applications, with hard requirements being:

* Sending state for a specific set or all known users to a defined set of local and remote users.
* The ability to trigger an initial sync for specific users, so they receive all current state.