[WIP][Awaiting developer action/feedback] Create merge request when editing on web UI.

Fixes ACCEPTING MR at: http://feedback.gitlab.com/forums/176466-general/suggestions/5350102-option-to-make-a-merge-request-when-editing-via-we

If user has push permission:

![screenshot from 2014-07-06 19 26 35](https://cloud.githubusercontent.com/assets/1429315/3489513/06e1cd86-0533-11e4-9246-af6e75b13b1d.png)

After clicking the fork checkbox the branch name input appears:

![screenshot from 2014-07-06 19 27 02](https://cloud.githubusercontent.com/assets/1429315/3489515/14123d88-0533-11e4-8e65-30210d14bd34.png)

If the user does not have push permission, the checkbox is checked and disabled:

![screenshot from 2014-07-06 19 27 41](https://cloud.githubusercontent.com/assets/1429315/3489516/1bfa8280-0533-11e4-811c-e1988c7ae1ee.png)

If this is OK I'll continue.