Removed old placeholder models for user groups.
Add Pod and PodMembership models.
Fix migration merge issues.

Create a new migration on top of merged-in migrations for plantshare.
Switch the foreign key for Pods to UserProfile instead of User, so we
can add convenience functions for retrieving Pods.
Add checklist models.
Add item fields and sharing fields.
Add methods for common checklist / sharing scenarios.
Merge branch 'feature/checklists'