Issue 2750: Allow mods to disable anonymous prompts

Resolves issue: http://code.google.com/p/otwarchive/issues/detail?id=2750

Allows moderators to disable anonymous prompts in their challenges. This simply hides the 'Semi-anonymous?' ticky on the prompt form from future use. It doesn't change the actual [code]:anonymous[/code] setting, and so won't break anonymity.