Implement FilteringService

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Use FilteringService for Dashboard, Group pages

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Specs for FilteringService

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Use FilteringService for project issuus, mrs

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Remove unnecessary contexts

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Replace hints with text that is obviously not valid.
Remove unnecessary spec
FilteringService works with scope :all event if current user is not defined

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
New branch doesn't exist yet.
 Merge branch 'refactoring/issues_filter' into 'master'

Refactoring: Issues/MR filtering logic

Move all issues, mr filtering logic into FilteringService
 Merge branch 'new-branch-usability' into 'master'

Improve usability of new branch form