Allow comments on closed/completed items

To use this effectively as a bug tracker, the users need to have the ability to post comments to submissions that are already closed by the admin.

Since in a few cases it has been so that a bug has been "fixed" but actually it wasn't and the user cannot post additional comments to the report/submission. Only choice is to, unnecessarily, open a new submission (which most probably won't bother in that case).

We used a forum as bug tracker before and there it worked well, so no reason to lock the submission upon closure.