Moved all the logic out of the template into a separate endpoint. This
makes it easy to extend to also support images from different sources
like LDAP/SAML/OIDC. Session-based caching is hard to do, so to allow
time-based caching in the browser, the url needs to be unique for every
user by using a query parameter.
Replaced the default/fallback user image with a new one. It is based on
the old one, but does not need css to be visible. And removed said css.
Gravatar has now its own setting named `gravatar_enabled`, which is
disabled by default.
