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.
CSRF has been initialized *before* the app config was fully read. That
made it impossible to configure CSRF properly. Moved the CSRF init into
the routes module, and switched from programmatic to decorated
exemptions. GET routes don't need to be exempted because they are by
default.
