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.
