Refactor the rate limiting code

Rate limiting was already redesigned to use Python limits. This
introduced some unexpected behavior, including the fact that only
one criteria is supported per limiter. Docs and setup utility are
updated with this in mind.

Also, the code was made more generic, so limiters can be delivered
for something else than authentication. Authentication-specific
code was moved directly to the authentication routine.

core/admin/mailu/utils.py

@@ -20,7 +20,7 @@ def handle_needs_login():
     )
 
 # Rate limiter
-limiter = limiter.Limiter()
+limiter = limiter.LimitWraperFactory()
 
 # Application translation
 babel = flask_babel.Babel()