Adding an option to configure session expiration time

Adds an expire_after option to gitlab.yml to centralize configuration of session expiration.

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

(I'll also be submitting a merge request to omnibus so it can also be easily configured through gitlab.rb)