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.
revert using the extension of the blob to determine the syntax highlighting language

nohighlight functionality for a hardcoded set of filenames is kept
HipChat service: correct service name & use v2 API

HipChat refers to their own product camel cased so we should do the
same. HipChat no longer recommends people use the deprecated v1 API so
switch to using the v2 API by default. hipchat-rb does not yet default
to v2 in any version so it must be specified.
HipChat service: support custom servers

HipChat allows users to run their own private servers and to be able to
support those we must connect to the correct URL when using one of these
custom servers.
Fixed an alignment issue, fixes #778
The blog post will trigger a mail to the list
add preliminary 7.6 upgrade guide

Add preliminary 7.6 upgrade guide. Makes it easier to add upgrades as changes are made rather than trying to round up everything at RC1.

Initial additions:
* Nginx changes needed again in 7.6 as they did not make the final 7.5 upgrade guide
* Suggest that user sets time zone (added in https://github.com/gitlabhq/gitlabhq/pull/8015 but missed in final 7.5 upgrade guide)

Replaces https://github.com/gitlabhq/gitlabhq/pull/8124
Formatting and sequence of contrubution paragraphs.
Definition of done added to the docs.
Fix spelling mistake, thanks Ewoud.
Update time zone rake task for production. Resolves #8387
Merge pull request #8394 from robertd/update-time-zone-in-gitlab-yml

Update time zone rake task for production. Resolves #8387