Settings v3: Feature flag changes

[turt2live]

Requires element-hq/element-web#14986
Requires element-hq/element-desktop#135
Fixes element-hq/element-web#10360

This is probably reviewable commit-by-commit, though it might be best to re-read the proposal and look at the code side-by-side.

A while ago we came up with a proposal for how to handle feature flags going forward, having gone through a gauntlet of trying to turn a feature on by default. The proposal described there is what this PR implements.

The theory is that by ensuring the labs section is always readily available we solve several problems we've had with the system:

1. We no longer have to remember to drop a feature_name: labs in a billion places.
2. We can force enable or disable a feature on our instances (likewise for self-hosted instances).
3. We can ensure the ecosystem is able to test a feature more broadly by enabling something by default for all instances as a pre-release step.
4. We can stop treating feature flags as special and instead convert them to regular settings as time progresses, if needed.

[turt2live]

I think the end to end tests are a false alarm?

Synapse log file at /tmp/tmp.07unFsTa5c
--
  | Starting ...
  | error starting (exit code: 1); see above for logs
  | No running worker of synapse.app.homeserver found (from /workdir/matrix-react-sdk/test/end-to-end-tests/synapse/installations/consent/env/bin/homeserver.pid)
  | The process might be managed by another controller (e.g. systemd)
  | handle_error

Looks like a Synapse problem

[jryans]

Looks great overall, thanks! 😄