Give a user a warning

[cquencer]

Reported by Svein W.

This is an important part of running a forum and I hope we can get it as soon as possible.

• It needs to use a PM system. Under normal conditions, a first warning should not be given in public.
• We need a history of warnings displayed on a users profile page. It only needs to be shown for moderators and admins, so it need to tie in with the ACL.
• The user must be able to answer the warning (explain, apologize etc.)
• Could also be a good thing to show an icon in the forum under the avatar (again - only for logged in moderators and admins) that shows a warning count.