Add Invitable interface

[rsmithlal]

We need to add a new module to contain the behaviour and relationships for invitable classes.

Tasks

• Create a new Inviteable module
• Instance methods
  • invite - Called on an instance of an inviteable record. Takes an identity as an argument, with an optional role
  • uninvite - Takes an identity as an argument

Relationships

• has_many invitations
• has_many invitees - through invitations