Fix bug that caused ajax-request to override values written to the session

Authentication/Session.php and its Subclasses do not have a open/closed -state
anymore. Read will refresh the session, write will always write the changes,
and opening/closing will be handled internally.

refs #5101
Fix messages in ConfigController

Store messages in the current user session to be able to fetch messages from
other controllers, so that the use can be redirected back to the index, instead
of staying in the original action

refs #5100
Fetch and clean user messages lazily, to ensure that only shown messages are removed

This will make the code of the ConfigController way easier, as messages can be
send from everywhere and there is no need to consider consquences of
redirections.

refs #5100
Merge branch 'bugfix/ajax-requests-override-session-5101'