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.
Merge branch 'upstream-master' into markdown-preview

Conflicts:
	spec/routing/project_routing_spec.rb
Merge pull request #8052 from mr-vinn/markdown-preview

Enable Markdown preview for issues, merge requests, milestones, and notes