Change user to polymorphic belongs_to
Setup dummy app to use polymorphic user
Update message unit test to use User class
serialize draft_recipient_ids after all
Serialize draft_recipients to/from draft_recipient_ids
Use draft_recipients* only for not-yet-sent messages

recipients will be used only for sent messages
Map .all onto all discussable classes for recipients list
Serialize and deserialize draft_recipients as json for form
Add fallback recipients if message doesn't have a parent and is unsent
Always reject the current_user
alias recipients= to draft_recipients=
Move recipient helpers into application controller

They may be used when embedding a form in a mailbox, for example
Merge pull request #1 from brentonannan/feature/polymorphic_users

Polymorphic users