Add support for inline videos in issue, MR and notes (on issue, commit, MR, and MR diff)

## What does this MR do?

It adds support for inline videos in issue, MR and notes (on issue, commit, MR, and MR diff). Most of the work was done by @hayesr in !3508 but a few improvements were still missing.

## Why was this MR needed?

To be able to play uploaded videos in GitLab!

## What are the relevant issue numbers?

Closes #4142.

## Screenshots

### Video players

![Screen_Shot_2016-07-19_at_18.44.09](/uploads/e85e531b455a41c3e66b26b356abaafd/Screen_Shot_2016-07-19_at_18.44.09.png)

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![Screen_Shot_2016-07-19_at_18.44.29](/uploads/05f52a812760210d1eae86a7f8fc48bc/Screen_Shot_2016-07-19_at_18.44.29.png)

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## 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)
- Tests
  - [x] Test `VideoLinkFilter`
  - [x] Test in `spec/features/markdown_spec.rb`
  - [x] Improve `spec/uploaders/file_uploader_spec.rb`
  - [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)

See merge request !5215

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

Introduced into the API in e80e3f53

[ci skip]

Also the specs have a minor improvement. Mainly the access right spec.
Changes are reflected in the docs

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

This standardizes all the project API formats. Also needed to support Huboard.

Fixes #4112

This is needed to support Huboard and a generally useful value.

has no branches to start.

Closes #3937

* add note to Events API
* add author section to Events API
* add noteable_id and noteable_type to Notes API
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

- Enable CI by default for all new projects

Updated projects.md to show tag_list field when performing GETs
Updated projects_spec.rb to include check for tag_list key in project list
Added changes to the CHANGELOG

* Impl Project#avatar_url
* Refactor ApplicationHelper: Use Project#avatar_url
* Update changelog

* add git_http_url and git_ssh_url to project web hook
* add visibility_level to project web hook
* add documentation about project visibility_level in API