#315325 by nbz, litwol: Userblocking architecture + other fixes. Early commit to enable the developers to focus changes that need to happen rather than the whole new feature. This commit introduces number of bugs and inconsistent $form_values[author] setting (sometimes uid and sometimes object), that needs to be fixed. also fixed message previews (feature broken partially due to inconsistent $form_values[author] setting). some other fixes.

#311541 by nbz, mrtoner, litwol: Freeze privatemsg db schema + msg delete functionality + changes to query builder