Remove feature flags for reactions and edits

[jryans]

Once we're confident that reactions and edits are stable, we can remove the feature flags.

[jryans]

We've decided it's best to handle both feature flags and unstable URLs together: a feature should be allowed to use to unstable APIs while it has a feature flag, but it should change to specced and versioned APIs when the flags are removed.

[bwindels]

Seems like quite a bit away from being able to do this, with backend and spec work stalled. Removing phase tag for now.

[jryans]

Reducing scope back down to just the feature flags. It turns out that we don't have a clear system to keep a feature flag in the code and default it on without config work for deployments, so it's best to remove the flags now to get the features out to everyone.

Filed a new issue #10361 for the future work around unstable URLs.

[jryans]

Merged to React SDK 1.4.0 release branch in matrix-org/matrix-react-sdk#3233