Django-mailinglist

Added
- Link to manage subscriptions page from the unsubscribe page
Removed
- Some unnecssary code for dynamically importing modules
Fixed
- Documentation reference to management command

Added
- Demo of pluggable user model to `test_project`.
Fixed
- **BREAKING!** Pluggable user model. This requires that you roll back your migrations for this app and reapply them.
- Project URL in setup.py.

Added
- Models graph to documentation
- Docs badge to readme

Added
- `MessageAttachment` model and machinery to manage and send attachments.
- `List-Unsubscribe` header to outgoing messages.
- Lots of documentation!
- Contents to `message_subject.txt` template.
- Management command, celery tasks for processing published submissions.
- View and URL for index of mailing list archives.
- Documentation generation within CI pipeline
Changed
- More fiddling with the badges
- Message preparation pipeline to reduce template lookups
Removed
- `Message.subject`

Changed
- Readme to point to correct github workflow status

Added
- Nothing, just getting CI pipeline working