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.
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
