Backport SmartInterval, PrettyTime, SubbableResource from EE.

## What does this MR do?

Backports infrastructure used for EE-only Timetracking so it can be used and improved upon in CE. 

This doesn't really need review... it was already reviewed and merged in EE. 

There are no side effects or conflicts, just three new classes added:

1. `SubbableResource` -- pubsub for ajax resources
2. `SmartInterval`-- for configurable polling
3. `PrettyTime` -- time parsing and formatting utility methods

- [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?

https://gitlab.com/gitlab-org/gitlab-ee/issues/985

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/870

cc: @jschatz1

See merge request !7573

adds dicussion icon and color change in system note links

adds discussion icons and sticky note icon for other system notes for now

fixes scss lint error

adds faded commit lists

hides first paragraph in commit list box

css tweak for commit list system notes

adds commit-list toggle functionality, css tweaks and css classnames more readable

small css fix in header. makes links bold in system note

renames class no-shade to hide-shade

adds entry for this merge request in changelog

removes commented line

removes the avatar-icon from discussion header

minor css tweaks to make the commit list alignment with header text

uses monospaced font to make the commit list lined up with all

removes icon from system note and align bullet list

resolves scss lint warings

adds helper function to extract system note message from first p tag

adds helper functions to check commit list count and haml cleanup

adds changelog entry under 8.14

adds changelog entry with changelog cli

removes helper and regex and makes commit list li count using JS

makes link in system note normal

brakeman build failure resolved

fixing rspec test based on new design for discussion

shows system note in lowercase

removes extra spaces from comments

adds code commenting for functions

adds semi-colon in some lines

fixes rspec given when merge build success

removes commented codes

rewrite changelog yml file

moves isMetaKey to common utils file

fixes some indentation issues

removes unnecessary variables and resolves some discussions

replaces jQuery parent function with siblings

fixes scss issues and variable spelling mistake

uses constant rather using hardcoded number for visible li count in long commit list

makes system note header all lowercase

uses color variables and adjust gradient a little

some minor changes for adding css classes

renames functions name for readability

changes changelog title

minor scss newline changes

makes system note less intrusive to a conversation

Moved most of the functions that contained "window.doSomething" that were located at:
- app/assets/javascripts/application.js
To the following file:
- app/assets/javascripts/lib/utils/common_utils.js

The functions listed here:
- window.ajaxGet
- window.split
- window.extractLast
- window.rstrip
- window.disableButtonIfEmptyField
- window.disableButtonIfAnyEmptyField
- window.sanitize
- window.unbindEvents
- window.shiftWindow

Now will be accessible from the "gl.utils" namespace