Kerberos support

https://dev.gitlab.org/gitlab/gitlabhq/issues/1414

Ability to login using kerberos credentials

![Screen Shot 2014-12-12 at 21.06.01](https://dev.gitlab.org/uploads/gitlab/gitlab-ee/cdc0fb7ee9/Screen_Shot_2014-12-12_at_21.06.01.png)

Ability to attach kerberos account

![Screen Shot 2014-12-12 at 21.19.23](https://dev.gitlab.org/uploads/gitlab/gitlab-ee/e926fb3798/Screen_Shot_2014-12-12_at_21.19.23.png)

See merge request !267

Update changelog 7.6

See merge request !1327

Kerberos dependency

See merge request !1324

Fix note update

In some strange cases Ruby thinks `system` is not AR field but
Kernel.system call and raises exception during save. It should prevent
this random wierd behaviour.

Exception log:

```
ArgumentError (wrong number of arguments (0 for 1+)):
  app/models/note.rb:505:in `system'
  app/models/note.rb:505:in `editable?'
  app/controllers/projects/notes_controller.rb:33:in `update'
```

See merge request !1326

Update changelog

keep in mind that there is unmerged MR without updating changelog https://dev.gitlab.org/gitlab/gitlab-ee/merge_requests/268

See merge request !269

In some strange cases Ruby thinks `system` is not AR field but
Kernel.system call and raises exception during save. It should prevent
this random wierd behaviour
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Added process for green tests

Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>

Fixed a lot of already defined notices

Added Code of Conduct

Add missing webhook doc for tag event

Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>

remove extra upgrade details, add missing config

Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>

Fix typo in the README.md for docker

Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>

add details on backing up your SSH host keys

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

Validate and sanitize snippet file name

Fixes #1816

See merge request !1322

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

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

CE changes from upstream

See merge request !266

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

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

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

Fix shell commands in shell install

Fixes #1610

See merge request !1320