Allow owners to add editors to organization

[ehmatthes]

One of the main draws of OC is collaborative building of competency systems. This will start the collaborative process, by allowing owners to add multiple editors to their organizations.

This will need to evolve into a system where individual users request to be added as an editor; but will need to work out how to do that while maintaining privacy of organizations that are not yet public.

[ehmatthes]

Implementation

• [x] I believe this is really just adding a form element to organization_admin_edit.
  • [x] It may be worth moving m2m relationship to org-based, not user based. It's more a property of the org than of the user. Will require some significant refactoring, but probably result in cleaner code.
  • Every element in the hierarchy could have an editors relationship as well; could even be part of abstract class?
• [x] Taking care of #89 first.

[ehmatthes]

• [x] Move m2m user-org relationship to Organization.
  • [x] Add m2m relationship to Organization; migrate.
  • [x] Add owner of each org to org.editors.
  • [x] Remove m2m relationship from User; migrate.
  • [x] Address failing tests.
  • [x] Address issues that aren't raised by tests.
  • [x] write tests for these issues when possible (using response.content.decode()) (unnecessary, all of these issues were caught by existing tests!)
  • [x] ie showing orgs a user can edit, on profile
  • [x] organizations template may need modification
• [x] Allow org owners to add and remove users as editors of orgs.
  • [x] Can't remove an owner. Override form.save? For now, just verify it didn't happen.
  • [x] Write tests for this.