delay is in seconds
more legible code in session_store
Added `GitLab restart required` help block to session_expire_delay

settings

This feature was requested long ago:

http://feedback.gitlab.com/forums/176466-general/suggestions/4118466-ability-to-register-only-from-ceratain-domains

This MR is based off !253 but changed to use application settings and use wildcard strings
to give more flexibility in pattern matching. Regexps seemed overkill and easy to get wrong.

Only restrict e-mail addresses upon creation

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Add new global application settings for default project and snippet
visibility levels.

Fix bug where error messages from Dropzone would not be displayed on the issues page

Closes #1258

Add checkboxes to the application settings page for restricted
visibility levels, and remove those settings from gitlab.yml.

Add the form-control CSS class to the feature checkboxes on the
application settings page to fix the vertical alignment with their
labels.  Also add aria-describedby attributes to form controls that have
a help text block.