Decide on and document a convention for singletons

> The singleton pattern is a design pattern that restricts the instantiation of a class to one object. This is useful when exactly one object is needed to coordinate actions across the system. 

The simplest implementation uses an object literal to contain the logic.

```javascript
gl.MyThing = {
  prop1: 'hello',
  method1: () => {}
};
```
A live example of this is [GfmAutoComplete](https://gitlab.com/gitlab-org/gitlab-ce/blob/172aab108b875e8dc9a5f1d3c2d53018eff76ea1/app/assets/javascripts/gfm_auto_complete.js.es6)

Another approach makes use of ES6 `class` syntax.
```javascript
let singleton;

class MyThing {
  constructor() {
    if (!singleton) {
       singleton = this;
       singleton.init();
     }
      return singleton;
  }

  init() {
    this.prop1 = 'hello';
  }

  method1() {}
}

gl.MyThing = MyThing;
```
A live example of this is [Sidebar](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/javascripts/sidebar.js.es6)

Another functional approach to define Singleton using `Javascript Revealing Module Pattern` is like below
```javascript
/**
  *  1. It instantiates only a single object
  *  2. It is safe – it keeps the reference to the singleton inside a variable, which lives inside a lexical closure, so it is not accessible by the outside world
  *  3. It allows privacy – you can define all private methods of your singleton inside the lexical closure of the first module pattern
  *  4. No this keyword trap (no wrong context referring)
  *  5. No use of new keyword
  *  6. Easy to write test code
  */
//
const Singleton = (function () {
  // Instance stores a reference to the Singleton
  var instance;
  function init() {
    // Singleton
    // Private methods and variables
    function privateMethod(){
        console.log( "I am private" );
    }
    var privateVariable = "Im also private";
    var privateRandomNumber = Math.random();
    return {
      // Public methods and variables
      publicMethod: function () {
        console.log( "The public can see me!" );
      },
      publicProperty: "I am also public",
      getRandomNumber: function() {
        return privateRandomNumber;
      }
    };
  };
  return {
    // Get the Singleton instance if one exists
    // or create one if it doesn't
    getInstance: function () {
      if ( !instance ) {
        instance = init();
      }
      return instance;
    }
  };
})();

const singletonObj = Singleton.getInstance()

```

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

## What does this MR do? 

Creates a space for discussion and contribution for interested devs.

## Why was this MR needed?

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] All builds are passing
(http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

See merge request !6620

Fixing the issue of visiting a project fork url giving 500 error when not signed…

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/24302

See merge request !7392

Support subscribing to group labels

https://gitlab.com/gitlab-org/gitlab-ce/issues/23586

See merge request !7215

Allows to authorize chat user against GitLab.

This is needed for: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7438

See merge request !7450

Remove unused CI gems

See merge request !7522

Start to document how to code for CE with EE in mind

Closes #23740

[ci skip]

See merge request !7248

Signed-off-by: Rémy Coutable <remy@rymai.me>

[ci skip]
Signed-off-by: Rémy Coutable <remy@rymai.me>

[ci skip]
Signed-off-by: Rémy Coutable <remy@rymai.me>

Include author in assignee dropdown search

## What does this MR do?

Previously when you searched for the author in the assignee dropdown (if they aren't in the project) it did not actually include the author. This changes that so that when you search for the author it will correctly include the user.

## Screenshots (if relevant)

![Screen_Shot_2016-11-17_at_11.14.53](/uploads/5a13475d8ded459fd3b1ba6570897665/Screen_Shot_2016-11-17_at_11.14.53.png)

## What are the relevant issue numbers?

Closes #22905

See merge request !7526

500 error on project show when user is not logged in and project is still empty

## What does this MR do?

Aims to fix the 500 error when the project is empty and the user is not logged in and tries to access project#show

## Screenshots (if relevant)
When the project is empty and the user is not logged in we default to the empty project partial instead of readme.

![Screen_Shot_2016-11-11_at_22.54.21](/uploads/3d87e65195376c85d3e515e6d5a9a850/Screen_Shot_2016-11-11_at_22.54.21.png)

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [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 it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Closes #23990

See merge request !7376

JiraService: simplify url generation

Fixes #24085

See merge request !7307