3.7 Request for Feedback

### Improvements:

- Core: Spoon registry has been refactored out in favor of the Symfony DI container. See UPGRADE_3.7.md for more info.
- Core: Don't throw exceptions in production mode on non-existing files.
- Core: Implemented a cookie-bar, see http://www.fork-cms.com/blog/detail/the-cookie-bar for more information.
- Core: use correct/new Facebook-js-snippet.
- Users: more logical way of handling user-permissions, see #684.
- Content blocks: only grab needed fields, see #669.
- Core: better description for CKFinder maximum image size settings.
- Core: used namespaces, see UPGRADE_3.7.md for more info
- API: use isAuthorized() instead of authorize(), see UPGRADE_3.7.md for more info.
- Core: CommonCookie and CommonUri are now in the src/Common folder
- Core: unused function BackendModel::imageSave is removed in favor of generateThumbnails().
- Core: removed duplicate mailer code and make the mailer a service.

### Bugfixes:

- Correct amount of sample comments in blog
- msgSequenceSaved was missing from core installer.
- Core: Modified misleading text about CKFinder maximum image size setting.
- Share with linkedin, fixed double url encoding.
- Faq: getByTags did not work in backend.
- Blog: fixes an issue where an incorrect revision could be used instead of the most recent one, see #680.
- API: use DIRECTORY_SEPARATOR instead of hardcoded /, fixes #682.