#330419 by nbz, Berdir and dereine: Block empty messages + Alter function to add content to the view massages page + consistent use of $User object in forms.

Author testing: Test sending a message and receiving sent confirmation. Recipient testing: Test receiving message sent and reading it.

Implemented tests to check access to inbox and write-new-message-form with and without read/write privatemsg permissions

#324406 by markDrupal, Berdir, nbz: refactoring of privatemsg_list (was: Make use of theme() functions in the inbox)

#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.

#321136 by nbz: implementing correct error message when trying to view a non existent message thread

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

 #297459 by mrtoner: Include create new message, inbox, and sent folder links in private messages block