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/static/custom/css/custom.css

@@ -42,15 +42,6 @@ table td {
     background-position: center;
 }
 
-.navbar-nav>.user-menu>.dropdown-menu>li.user-header>img.img-circle.offline {
-    filter: brightness(0);
-    border-color: black;
-}
-
-.navbar-nav>.user-menu .user-image.offline {
-    filter: brightness(0);
-}
-
 .search-input {
     width: 100%;
 }