The registration fallback is broken and unspecced. This removes it
since there is no plan to spec it.
Note that this does not modify the login fallback code.
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 converts a few more of our inline HTML templates to Jinja. This is somewhat part of #7280 and should make it a bit easier to customize these in the future.
