Rework user image handling

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.

powerdnsadmin/models/setting.py

@@ -193,7 +193,8 @@ class Setting(db.Model):
         'otp_force': False,
         'max_history_records': 1000,
         'deny_domain_override': False,
-        'account_name_extra_chars': False
+        'account_name_extra_chars': False,
+        'gravatar_enabled': False,
     }
 
     def __init__(self, id=None, name=None, value=None):