In-memory Celery backend for tests

[dato]

The use of Redis is avoided in tests by mocking the .delay method of relevant Celery tasks. An alternative would be to run tests with an in-memory Celery backend, and let task objects be created there.

This PR implements this, but doesn't touch any existing patch('...') blocks or decorators. From the moment it is merged, patch blocks would not be needed in new tests (and could be removed when editing existing ones).

• in-memory backend when running from pytest
• in-memory backend when running from manage.py
• check test suite run time if all mocks are dropped
  • I prepared a PR that removes now-unneeded mocking (Use in-memory celery for test suite instead of mocking dato/bookwyrm#23, to be submitted later), and did not notice any slow downs (perhaps even slightly faster)
• add an example worker test