Forcibly destroy existing session on logout.

To comply with security guidelines, we need to destroy the session when
a user logs out. This means that the session's key in the Redis cache
needs to be deleted. Flask expects to _always_ have a session object. If
the current session object does not exist in the Redis cache, Flask will
reserialize and store it at the end of the request. In order for
session deletion to work, we need to delete the key for the existing
session and then replace the session object with a new, empty one.

This also updates the SessionLimiter class so that the session prefix is
configurable.

config/base.ini

@@ -42,6 +42,7 @@ SECRET_KEY = change_me_into_something_secret
 SERVER_NAME
 SESSION_COOKIE_NAME=atat
 SESSION_COOKIE_DOMAIN
+SESSION_KEY_PREFIX=session:
 SESSION_TYPE = redis
 SESSION_USE_SIGNER = True
 SQLALCHEMY_ECHO = False