Extended tables

Originally, we kept revision tables of absolutely everything. However, this can quickly get out of control if some fields naturally receive frequent updates (in particular, user.lastSeen). I compromised on the following fix.

• In order to prevent countless revisions caused by the field table.field, move it to table_ext.field.
• Extension tables do not have revision tables (so no revision_table_ext).
• Extension tables do not have createDate, modDate and modUserId fields.
• Extended fields should not need to be cloned (for example, during pending edit creation).
• Extended fields will be saved individually, for example to update a user's last seen timestamp. Specifically, extended fields are not saved when the object is edited.