Factor new issue and edit MR forms.

There was a lot of code duplication here.

There is also a third copy which has diverged more and which we should DRY out as well: the new MR form. It probably has the best visual style of the three and should be the one to copy. Opened an issue for that at: https://github.com/gitlabhq/gitlabhq/issues/7679 so we won't forget.

The issue edit and new were already factored.

Minor visible changes:

- "Issue is parsed with GFM" message changed to: "Parsed with GFM". Shorter, DRYer, and it is obvious that we are talking about the description that corresponds to the textarea above.

I cannot see any other behavior / UI changes.

Non-visible changes:

- title input element:

    - remove rows attribute. Does nothing on `input type="text"`.
    - remove class `"pad"` from the merge request form. Not used anywhere in the app.

TODO in future merge requests: DRY those forms even further on the points they diverged more:

- errors
- labels
- submit

Found using https://github.com/UncleGene/flay-haml
@cirosantilli Is this PR still WIP?