Issue #1089290 by Berdir: Added default and variable_get() fallback and migration for old pm disable settings.

#979956 by Berdir | IWasBornToWin: Fixed tagging messages on view thread page doesn't work with a %user url prefix.

#864554 by Berdir, shenzhuxi: Added settings to control the displays of input format selection and preview button.

#850022 by Berdir | BenK: Improved autocomplete for different recipient types and name conflicts between them.

#850552 by Berdir | BenK: Fixed URL for non-existent users shows messages for current logged in user.

#555016 by Berdir | neriweaver, trupal218, BenK, nbz, archard, nilsja, kriskd, Liliplanet, yaz085, litwol, Feet, jaron: Added recipient types, allowing to send messages to roles and other recipient groups.

#778372 by Berdir | carlescliment: Fixed missing permission check when a user has disabled privatemsg

#734900 by Berdir | robby.smith: Added permission to allow disabling privatemsg and display notice when disabled.

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