Documentation on CI triggers

Closes #3432

## Notes

* Implement build trigger API - https://gitlab.com/gitlab-org/gitlab-ci/merge_requests/229
* Build trigger API - https://gitlab.com/gitlab-org/gitlab-ci/issues/257
* Build pipeline - https://dev.gitlab.org/gitlab/gitlab-ci/issues/282 and https://gitlab.com/gitlab-org/gitlab-ce/issues/3743
* Dependent builds - https://dev.gitlab.org/gitlab/gitlab-ci/issues/328
* Travis docs - https://docs.travis-ci.com/user/triggering-builds/
* Custom variables Circle CI example - https://circleci.com/docs/nightly-builds
* Triggers API (CI) will be done in a separate MR
* Notify https://gitlab.com/gitlab-org/gitlab-ci/issues/368 once done

## Docs needed to change

- [x] `doc/ci/README.md`
- [x] `doc/README.md`

See merge request !2161

Add API support for looking up a user by username

Needed to support Huboard

See merge request !2089

Require two factor authentication

Allow Admins to force 2FA on their users, as discussed here: gitlab-org/gitlab-ce#3783

See merge request !2155

Escape all the things.

See merge request !2209

simplified code and fixed stuffs

Enable "Add key" button when user fills in a proper key

Closes #4295

See merge request !2208

Closes #4295

[ci skip]

Move changelog items to their correct place.

Fixes #4236

See merge request !2201

Fix identity and user retrieval when special characters are used

Fixes #4023

I also added tests to make sure the user with special characters in his name is returned correctly.

@rspeicher this probably should be added to 8.3 as a patch.

See merge request !2176

Emoji picker: add "frequently used" category

depend on !2172

![Screen_Shot_2015-12-24_at_16.39.04](/uploads/7117173c6f519282054bcb1b7ba248f4/Screen_Shot_2015-12-24_at_16.39.04.png)


See merge request !2180

Needed to support Huboard

Avoid allocations in Ability class.

Fix explore projects page UI

# Before

![Screenshot_2015-12-24_14.13.03](/uploads/fdae7648acc8b8076709e4bfc867a5f8/Screenshot_2015-12-24_14.13.03.png)


# After

![Screenshot_2015-12-24_14.11.29](/uploads/b1ecd3bebfadab9d2ceabaae27046271/Screenshot_2015-12-24_14.11.29.png)



cc @jschatz1 @skyruler 
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>


See merge request !2202

Fix NewBranchForm spec

cc @vsizov 

See merge request !2204

Dropdowns for clone. Star and Fork with new count areas. Star and Unstarred Works now

Fixes #2876 and #3482
Used existing styles. <br/>
Did not attempt to make pixel perfect as styles already exist. <br/>

![Screen_Shot_2015-12-18_at_5.24.56_PM](/uploads/a79ba1e0be0232f2aeef8a43f3253479/Screen_Shot_2015-12-18_at_5.24.56_PM.png)

See merge request !2156

Emoji picker search

depend on !2172

Issue https://gitlab.com/gitlab-org/gitlab-ce/issues/3576

!!! Should be merged after https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2198

![Screenshot_2015-12-23_11.44.57](/uploads/01da0b62c3c234307142e2c4db68e53f/Screenshot_2015-12-23_11.44.57.png)

See merge request !2186

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

Fix Error 500 when global milestones have slashes

* Closes #4226

* Closes https://github.com/gitlabhq/gitlabhq/issues/9921

See merge request !2182