Send conversation invitations on content .save()

[mindey]

Right now, we have conversation invitations in detail views. We would like to enable the user to invite other users directly after they compose the Goal/Idea/Plan/etc.

So, let's start with this:

1. Add field Goal.invite_emails (it could be a text field, or EmailListField??)
2. Add widget with validation.
3. Upon Goal.save(), for all e-mails that were not yet invited or not a user, send conversation invite with username before @ mark, plus increasing number, and invitation_text='Thanks!'.