Add Open Graph meta tags



See merge request !2192

Adding  CRIME Vulnerability to Security doc

Added an additional page to the security section that goes over current status and fixes for the TLS Protocol CRIME Vulnerability. Also added its respective link at the security README. 

Any suggestions appreciated @JobV @sytses 

See merge request !2102

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