This PR changes http-based image URLs to be https in html templates.
This impacts the Synapse SSO error page, where browsers report mixed
media content warnings.
Also, https://matrix.org/img/vector-logo-email.png is currently broken
but the URL has been updated to be https anyway.
Signed-off-by: Ashish Kumar <ashfame@users.noreply.github.com>
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.
