We might have events in the staging area if the service was restarted while there were unhandled events in the staging area.

Fixes #10295

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

Instead of mixing them with user authentication methods.

this was a typo introduced in #10282. We don't want to end up doing the
`replace_stream_ordering_column` update after anything that comes up in
migration 60/03.

Now that we've fixed it.

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).

We need to rebuild *all* of the indexes that use the current `stream_ordering`
column.

This broke in #10272

Fixes #9490

This will break a couple of SyTest that are expecting failures to be added to the response of a federation /send, which obviously doesn't happen now that things are asynchronous.

Two drawbacks:

    Currently there is no logic to handle any events left in the staging area after restart, and so they'll only be handled on the next incoming event in that room. That can be fixed separately.
    We now only process one event per room at a time. This can be fixed up further down the line.

This adds a simple best effort locking mechanism that works cross workers.

Previously m.child.room events in non-space rooms would be
treated as part of the room graph, but this is no longer
supported.

It was possible for us not to find any rows in a batch, and hence conclude that
we had finished. Let's not do that.

* Move background update names out to a separate class

`EventsBackgroundUpdatesStore` gets inherited and we don't really want to
further pollute the namespace.

* Migrate stream_ordering to a bigint

* changelog

Currently when a new build of the docs is created, an `index.html` file does not exist. Typically this would be generated from a`docs/README.md` file - which we have - however we're currently using [docs/README.md](https://github.com/matrix-org/synapse/blob/394673055db4df49bfd58c2f6118834a6d928563/docs/README.md) to explain the docs and point to the website. It is not part of the content of the website. So we end up not having an `index.html` file, which will result in a 404 page if one tries to navigate to `https://matrix-org.github.io/synapse/<docs_version>/index.html`.

This isn't a really problem for the default version of the documentation (currently `develop`), as [navigating to the top-level root](https://matrix-org.github.io/synapse/) of the website (without specifying a version) will [redirect](https://github.com/matrix-org/synapse/blob/a77e6925f26597958eccf0ef9956cb13c536e57e/index.html#L2) you to the Welcome and Overview page of the `develop` docs version.

However, ideally once we add a GUI for switching between versions, we'll want to send the user to `matrix-org.github.io/synapse/<version>/index.html`, which currently isn't generated.

This PR modifies the CI that builds the docs to simply copy the rendered [Welcome & Overview page](https://matrix-org.github.io/synapse/develop/welcome_and_overview.html) to `index.html`.

Rather than persisting rejected events via `send_join` and friends, raise a 403 if someone tries to pull a fast one.

The idea here is to stop people sending things that aren't joins/leaves/knocks through these endpoints: previously you could send anything you liked through them. I wasn't able to find any security holes from doing so, but it doesn't sound like a good thing.

This implements refresh tokens, as defined by MSC2918

This MSC has been implemented client side in Hydrogen Web: vector-im/hydrogen-web#235

The basics of the MSC works: requesting refresh tokens on login, having the access tokens expire, and using the refresh token to get a new one.

Signed-off-by: Quentin Gliech <quentingliech@gmail.com>