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 attempts to be a direct port of https://github.com/matrix-org/synapse-dinsic/pull/74 to mainline. There was some fiddling required to deal with the changes that have been made to mainline since (mainly dealing with the split of `RegistrationWorkerStore` from `RegistrationStore`, and the changes made to `self.make_request` in test code).
Germain