ApplicationSetting.current was called in every pages, cache it and expires it after it updated.

This changes will avoid a SQL query in every pages (~0.3 - 0.5ms).

```SQL
SELECT  "application_settings".* FROM "application_settings"   ORDER BY "application_settings"."id" DESC LIMIT 1
```

This option only existed to ease the CI-to-CE/EE migration process.

This commit partially reverts 8b05abe8

Closes #1798

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

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

Check for nil values in the restricted_visibility_level validation
method, and set the restricted visibility request parameter to `[]` when
it's missing from the request.

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