Version 0.5.0.rc

Release candidate for the first syncronised leap release 0.5.0.

* do not allow to clear pgp key
* simple format validation for pgp keys
* respond to /keys/:login with text
* service levels introduced to the configuration
* more robust session store
* render landing page and basic forms without connection to couch
* configurable set of actions for users and admins
* refactoring: remove overview controller - use Users#show instead
* prevent /login from crashing when logged in already
* remove actions from ui that currently break the client
* format ticket comments properly
* enable billing engine by default
* consolidate APP_CONFIG[:billing]
* fix a regression where overview link was not active anymore