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
update order of upgrade guide

GitLab needs to be stopped when backup is took.
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.
Fix typo in the README.md for docker

The container name has been previously renamed to "gitlab_app".
Wait 15 minutes before Sidekiq MemoryKiller action
Merge branch 'upstream-master' into markdown-preview

Conflicts:
	spec/routing/project_routing_spec.rb
Drop vagrant preference in issue tracker guidelines.
Use constants instead of getters
Merge branch 'tracker_guidelines' into 'master'

Drop vagrant from issue tracker guidelines

Is this the preferred method of reproducing an issue? If yes, disregard this MR.

\cc @sytses @marin

See merge request !255
Fix typo
Use the new SIDEKIQ_MEMORY_KILLER_MAX_RSS variable