Factor issue and edit MR form label and submit.

This is a direct continuation of: https://github.com/gitlabhq/gitlabhq/pull/7678

I had left it out because they were harder to factor, but I think I've manged now.

There should be no UI changes.

Implementation notes:

- this works because `link_to [issuable.project, issuable]` is magic and redirects to either index or show depending on if `issuable` already exists or not.

- that `issuable.new_record?` is never reached for the Merge request because the new issue and new MR have diverged... but let's leave that in place for when we merge them together.