Use a base template to create a cohesive feel across the HTML
templates provided by Synapse.
Adds basic styling to the base template for a more user-friendly
look and feel.
This is a combination of a few different PRs, finally all being merged into `develop`:
* #5875
* #5876
* #5868 (This one added the `/versions` flag but the flag itself was actually [backed out](891afb57cb (diff-e591d42d30690ffb79f63bb726200891)) in #5969. What's left is just giving /versions access to the config file, which could be useful in the future)
* #5835
* #5969
* #5940
Clients should not actually use the new registration functionality until https://github.com/matrix-org/synapse/pull/5972 is merged.
UPGRADE.rst, changelog entries and config file changes should all be reviewed closely before this PR is merged.
Sends password reset emails from the homeserver instead of proxying to the identity server. This is now the default behaviour for security reasons. If you wish to continue proxying password reset requests to the identity server you must now enable the email.trust_identity_server_for_password_resets option.
This PR is a culmination of 3 smaller PRs which have each been separately reviewed:
* #5308
* #5345
* #5368
