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.
The init script was pushing an application context, which maked
flask.g global and persisted across requests. This was evaluated
to have a minimal security impact.
This explains/fixes #738: flask_wtf caches the csrf token in the
application context to have a single token per request, and only
sets the session attribute after the first generation.
