Add documentation for the Sidekiq MemoryKiller

We already merged the code, and we are already using it on gitlab.com. This MR adds documentation.

See merge request !1325

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

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>

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

Fix shell commands in shell install

Fixes #1610

See merge request !1320

Dropdown icon not clickable

Fixes #1642 by using `pointer-events` property supported by [modern browsers](http://caniuse.com/#search=pointer-events)

See merge request !1319

Show files named diff

Fixes #1749

See merge request !1318

Add an extra reference to the 'shell commands' guide

The 'shell commands' guide now also has tips on opening/reading files and handling paths.

See merge request !1321

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