Allow moderators and administrators to be warned.

AdminCP access can obviously bypass any warning issued, and warnings themselves can be edited or deleted depending on permissions for the user.

Introduces 7 new permissions:

• Give moderators warnings directly.
• Give administrators warnings directly.
• Delete all warnings for Moderators.
• Delete all warnings for Administrators.
• Prevent posts being warned.
• Prevent being directly warned.
• Prevent profile posts being warned.

Performance

Requires between 0 or 1 extra queries depending on if the active members on the content to-be-warned share a permission combination group with the visitor.