Issue: DM Classes Don't Support Platform Valid Fields

Some platforms may have extra fields for certain content that is typically added as another column to the existing table (e.g. XenForo has warning_id to correspond to an entry in the warning table), but currently it is not possible to add valid fields in the DM's platform sub-class, only via plugin.

It is easier to achieve this in XenForo, since a vw class is between the core DM and the content's platform DM, but in vBulletin the content's platform DM always extends directly from the core DM. In order to achieve this, an extra method for more_valid_fields needs to be defined in an intermediate class, and implemented in the main DM for each content type.

This refactor will also clean up a significant number of vBulletin platform DMs.