Clean up ci yaml doc
* Fix headings so that they are picked up by doc.gitlab.com
* Stick to 80 characters
* Clean up some examples
See merge request !2052
Update rerun to remove celluloid as dependency
After sidekiq 4 migration, we no longer need celluloid. `rerun` recent version also removed it from its dependency, so by getting this MR merged, we can solve #3797
See merge request !2088
Link to correct (source) project on MR builds tab
Used to link to `gitlab-org/gitlab-ce/commits/<branch-name>`, now links to correct project.
See merge request !2115
Fix headers of milestone and snippet show pages
Fix layout, and move "Edit" button to very right just like issue/MR.
Before:


After:


cc @skyruler
cc @rspeicher 8.3 please!
See merge request !2104
Fix right margin of control list
Before:

After:

See merge request !2114
Issuable sidebar tweaks
- Move cross-project reference down
- ~~Fix "Notifications" label color~~ Reverted
- ~~Use regularly colored button for "Subscribe"/"Unsubscribe"~~ Reverted
Before:

After:
(Only the moved cross-project reference is still relevant, other changes have been reverted)

cc @skyruler
cc @rspeicher 8.3 please!
See merge request !2105
Fix double right border in MR discussion
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
# Before

# After

cc @skyruler
See merge request !2113