Incuna-groups

- Add new email templates to MANIFEST.in.

- Fix urls.py import structure.

- Refactor `views.py` into several files in the `views` package. These modules are
`views._helpers`, `views.comments`, `views.discussions`, `views.groups`, and
`views.subscriptions`. You will need to edit any import statements you are making
to `incuna-groups` views. (No URLs have changed.)

- Use html `buttons` instead of `inputs` for form submission.

- Add the ability to reply to discussions by replying to notification emails.

- Add a test project for easier creation of migrations and testing the admin, as well as
to run the tests themselves.
- Hide `subscribers`, `ignorers` and `watchers` from the admin, since they aren't meant
to be editable there.
- Make all the `ManyToManyFields` non-required.