Add Todos

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/2425

Tasks:

-  Prepare database
 - [X] Create a new table (`todos`)
- Tasks Queue view
 - [X] Add a number icon showing the number of todos on the top right next to the new and logout button that will redirect the user to the todos page
 - [X] Add a chronological list of todos, with the 'Todos' tab active by default
 - [X] Add a 'Done' button to each todo
 - [x] Add filters (project, author, type, and action)
- Todos generation
 - [X] When user issue/mr is assgined to someone
 - [x] When user is mentioned on (issues/mr's/comments)
- Mark todo as `done`
 - [X] When clicks on the 'Done' button
 - [X] When edit issue/mr
 - [X] When left/edit a comment
 - [X] When reassign issue/mr
 - [X] When add/remove labels to issue/mr
 - [X] When issue/mr is closed
 - [X] When mr is merged
 - [X] When added an emoji
 - [X] When changed the issue/mr milestone

* Screenshot:

![Screenshot_2016-02-20_12.45.57](/uploads/4b2554b1bde25aed3347e1ae41e8e0c0/Screenshot_2016-02-20_12.45.57.png)

See merge request !2817

Fix bugs in MergeWhenSucceeded

1. This fixes support for merge when succeeded for statuses without ref.
2. This fixes support for merge when succeeded for multiple stages. Stages are created after all builds for previous one are finished.

Fixes: https://gitlab.com/gitlab-org/gitlab-ce/issues/9060 https://gitlab.com/gitlab-org/gitlab-ce/issues/8108 https://gitlab.com/gitlab-org/gitlab-ce/issues/12931 https://gitlab.com/gitlab-org/gitlab-ce/issues/13269

/cc @grzesiek @DouweM @rspeicher 


See merge request !2894

This ensures that the emptiness cache (used for Repository#empty? and
Repository#has_visible_content?) is flushed after comitting changes
(using the web editor, API or Git) for new repositories. Once a
repository is no longer empty there's no need to explicitly flush the
cache for Repository#empty?. The cache for
Repository#has_visible_content? in turn is already flushed whenever
needed.

Fixes gitlab-org/gitlab-ce#13387