~ Invite to discussion first iteration

[Ready]

TODO:
 - Test rendering of form from group page, subgroup page, discussion page
 - Test sending invitations from group page, subgroup page, discussion page
 - Test abilities re. invitations (can_invite) e.g. whether it is still in affect when inviting from a discussion page
 - Test receiving invitation, following the process through and check landing page - invite_to_group should land on group page after acceptance of invitation, invite_to_discussion should land on specific discussion page (page from which the invitation was sent) after acceptance of invitation
 - Invitation to a discussion should make invitee a member of the group
 - Test the mailers(!) 

   - inviting members of parent to a subgroup discussion or inviting existing Loomio users to a discussion should auto-add them to the group

...and anything else you can think of