Branch permissions layout

CE part of https://gitlab.com/gitlab-org/gitlab-ee/issues/179

Related javascript code has been refactored and also layout has been updated.

![Screen_Shot_2016-08-04_at_3.23.19_PM](/uploads/a0f6165e255b3f93dcb80eaa3f3318e4/Screen_Shot_2016-08-04_at_3.23.19_PM.png)


See merge request !5652

Add migration-related tips to the "Merge Request Guidelines" doc

## Why was this MR needed?

To avoid issues like #20606 cropping up again.

See merge request !5700

Document that webhook secret token is sent in X-Gitlab-Token HTTP header

## What does this MR do?

Note that the secret token is sent in the X-Gitlab-Token header on the webhook documentation page, as well as directly below the secret token field on the webhook settings form.

## Are there points in the code the reviewer needs to double check?

No.

## Why was this MR needed?

It took me a while to figure out how to verify the token in my hook endpoint. Issue #18256 is where I found how to do it.

## What are the relevant issue numbers?

#18256 

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)
Closes #18256

See merge request !5664

fix: respect data-attribute 'skip-users' in user_selects

Backport https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/604 to CE, so that JS file is the same in both versions. Thanks @hwdegroot!

No CHANGELOG needed because this code path isn't used anywhere in CE at the moment.

See merge request !5685

Simplify feature introduction note



See merge request !5705

[ci skip]

[ci-skip]

Add doc guidelines on documents naming and location

## What does this MR do?

Add guidelines on the structure of the documentation.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/3349

See merge request !5641

Add 'run tests' development docs from GDK

This adds some documentation that lived in
https://gitlab.com/gitlab-org/gitlab-development-kit but which belongs
more in here.


See merge request !5684

Add examples to repository files API

## What does this MR do?

Add examples to repository files API.

## Why was this MR needed?

There were no examples.

## What are the relevant issue numbers?

fixes #19779


See merge request !5465

Since .panel-heading is a bit different from bootstrap, using .panel-title for h3 looks different.

So with this .panel-title will look good again inheriting custom properties our app sets for .panel-heading

Since there’s only one dropdown of this type we don’t need a class to create this dropdown masively.

We should stop passing events as a parameter since we cannot call the method programatically without faking or creating an event even when that event is not entirely required.

Since there are only one dropdown of each type there are no need to create a class to initialize multiple elements with the same CSS class name.

Also updates protect branch list

Added tests to dateTime utility getDayName method

## What does this MR do?

It was missing tests so i've created some tests to check the correct day name is returned

See merge request !5324

Hope to avoid CHANGELOG conflicts. Credit myself for the MR.

Added changelog item for issuable form dropdowns

## What does this MR do?

Adds CHANGELOG item about the new dropdowns in the issuable form

See merge request !5266