Only show up or down votes icon on issue list if there are any
Closes#13649

See merge request !2909
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:

See merge request !2817
Award emoji supports retina displays
Based on discussion in !2800
There is now a retina spritesheet for emojis. It is 2mb, don't know how big an issue that is.
Before:

After:

Also updated the rake task to make the @2x spritesheet https://gitlab.com/snippets/15348
cc\ @rspeicher @vsizov
See merge request !2891
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.
Fixesgitlab-org/gitlab-ce#13387