Fix #1420 Ban page visible even if no allowed to edit ban.

Fix #1420 Ban page visible even if no allowed to edit ban.

- Also adds a ban message bit similar to the away bit.
- Removes one `<tr>` tag from the away bit template.
- Hides "Ban this user in Mod CP" from profile if no allowed to edit.
- Hides "Lift Ban" from the "Add Ban" page.

Downside? One extra query in profiles. That is why I added the new feature. Plugins may also benefit from it.

#1243
It is now private to only forum staff (same checks as in ModCP).