Fix html templates to load images only on HTTPS (#14625)

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>