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Optimize "cache_key" using a concern

## What does this MR do?

This MR adds a concern (used by Issue and Note) that provides an optimized version of Rails' `cache_key` method. See 77c8520e for more details.

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

No, though a spell check would be appreciated.

## Why was this MR needed?

When loading a lot of data from Redis (e.g. an issue with lots of notes) quite a large amount of time is spent in generating cache keys. This is due to multiple reasons such as:

* Rails trying to figure out if it should use `updated_at` or `updated_on` using somewhat inefficient code
* Rails relying on pluralization logic to figure out how to generate a cache namespace using a model name
* Rails calling a whole bunch of methods in general in the process of generating cache keys

In short, Rails is trying to cater to every possible use case, at the cost of performance.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/13651 is not directly related but I ran into this `cache_key` problem when looking into said issue.

See merge request !5715

Update templates

There was a copy pasta in the templates, therefor another MR with updates.

See merge request !5714

This concern provides an optimized/simplified version of the "cache_key"
method. This method is about 9 times faster than the default "cache_key"
method.

The produced cache keys _are_ different from the previous ones but this
is worth the performance improvement. To showcase this I set up a
benchmark (using benchmark-ips) that compares FasterCacheKeys#cache_key
with the regular cache_key. The output of this benchmark was:

    Calculating -------------------------------------
               cache_key     4.825k i/100ms
          cache_key_fast    21.723k i/100ms
    -------------------------------------------------
               cache_key     59.422k (± 7.2%) i/s -    299.150k
          cache_key_fast    543.243k (± 9.2%) i/s -      2.694M

    Comparison:
          cache_key_fast:   543243.4 i/s
               cache_key:    59422.0 i/s - 9.14x slower

To see the impact on real code I applied these changes and benchmarked
Issue#referenced_merge_requests. For an issue referencing 10 merge
requests these changes shaved off between 40 and 60 milliseconds.

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