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

This implements an accepted suggestion from the GitLab [feedback forum](http://feedback.gitlab.com/forums/176466-general/suggestions/5409046-markdown-preview-for-creating-issues)

I added the Write/Preview tabs from the note form to other areas so that Markdown previews are now available when creating and editing notes, issues, merge requests, and milestones.

This is similar to #6356, but that PR was closed and the source branch seems to be gone now.  @cirosantilli my apologies for duplicating your PR, but I've really been missing this feature in GitLab.

![edit_note_preview](https://cloud.githubusercontent.com/assets/2551678/4656940/86dbc116-54e0-11e4-9004-17dd29d2c676.png)

![new_issue_preview](https://cloud.githubusercontent.com/assets/2551678/4656946/94ea1898-54e0-11e4-9456-37e7b48866e7.png)

![edit_issue_preview](https://cloud.githubusercontent.com/assets/2551678/4656950/a38e26f0-54e0-11e4-9034-3188f38fba2d.png)

![new_mr_preview](https://cloud.githubusercontent.com/assets/2551678/4656956/ad72579a-54e0-11e4-9fde-0667ed23311f.png)

![edit_mr_preview](https://cloud.githubusercontent.com/assets/2551678/4656958/b4b68648-54e0-11e4-8b50-b2cd29d3f1b6.png)

![new_milestone_preview](https://cloud.githubusercontent.com/assets/2551678/4656959/c06d9a76-54e0-11e4-81bc-aa247406451b.png)

![edit_milestone_preview](https://cloud.githubusercontent.com/assets/2551678/4656960/c5df92ac-54e0-11e4-8045-01606e10b8ee.png)

Can we use GET here? Use safe methods when possible.