Make sure Markdown previews always use the same styling as the eventual destination.

CHANGELOG

app/assets/javascripts/notes.js.coffee

app/views/projects/_issuable_form.html.haml

app/views/projects/_md_preview.html.haml

app/views/projects/merge_requests/_new_submit.html.haml

app/views/projects/milestones/_form.html.haml

app/views/projects/notes/_edit_form.html.haml

app/views/projects/notes/_form.html.haml

app/views/projects/wikis/_form.html.haml

features/steps/project/merge_requests.rb

features/steps/shared/note.rb

spec/features/notes_on_merge_requests_spec.rb