Factor new issue and edit MR forms.

The code duplication here was huge.

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.

Minor visible changes:

- "Issue is parsed with GFM" 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 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