Add Markdown preview to more forms

Enable Markdown previews when creating and editing issues, merge
requests, and milestones, and when editing notes.
Use GET instead of POST for Markdown previews
Refactor Markdown preview tests

Create a new shared module for common issue/merge request behavior,
use `expect` syntax instead of `should`, and avoid `visible: false` in
the `have_css` matcher.
Fix Markdown preview tests

Update Spinach tests for Markdown previews for the new-ish tab UI that
replaced the old preview/edit toggle button.
Add new Markdown preview test

Add a test to make sure that Markdown previews are available when
editing an existing issue.
Fix long line in view
Fix houndci warnings
Fix more Markdown preview tests
Fix test assertions

Make sure we're asserting the correct thing when testing visible and
invisible DOM elements.
remove extra upgrade details, add missing config

Update 7.5 update guide:

* Remove unnecessary upgrade details (they were completed in 7.4 upgrade and only needed one time)
* Add missing Nginx config details
add details on backing up your SSH host keys

Users need to backup SSH host keys if they want to do a complete restore
to the same domain and not have users get the `WARNING: REMOTE HOST
IDENTIFICATION HAS CHANGED!` message.
Make docker image file user agnostic, to prevent confusion over official images.
Make the docker commands so that people can build their own images.
Change gitlab to gitlab_image on data run and add tail, thanks Vincent Robert.
Show username in comment header for easier mention

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Improve Monokai highlight style to match original

The current monokai style in highlightjs is not very true to the
original and lacks colors for certain syntactic items. This change's
goal is to bring the highlightjs monokai style in line with the original
design from http://www.monokai.nl/blog/2006/07/15/textmate-color-theme/
Merge branch 'username-in-comments' into 'master'

Username in comments

Its hard to mention person in comment if you don't know his username.
This patch show person username right after his name in comment row

See merge request !1281
Change twitter handle from gitlabhq -> gitlab
Merge pull request #8420 from Razer6/change_twitter_handle

Change twitter handle from gitlabhq -> gitlab
Disable Sidekiq arguments logging by default