- Patch #39135 by Zen: various contact form improvements/fixes.

  + Conversion to _validate + _submit model..
  + Split replaced by explode - faster
  + Fixed typos: Recipient
  + Fixed weight defaulting to -10
  + Popped mail subject formatting into a t()
  + Popped '--' formatting into a t()
  + Does a lot of documentation fixing/rewriting.
  + Renamed contact_user_mail form functions to contact_mail_user for consistency.
  + ...