Add session expiration delay configuration through UI application

Setting is accessible by the administrator through the UI and defaults to 1 week (the current setting)

Answers the following suggestions:

  * http://feedback.gitlab.com/forums/176466-general/suggestions/6210719-make-session-length-configurable
  * http://feedback.gitlab.com/forums/176466-general/suggestions/6730512-automatic-logout-after-a-time-being-idle

See merge request !774