Flask Celery Example and Docs Improvement

[michaelconan]

The flask documentation and repository examples provide some helpful information about using Celery with Flask, however, there are some core elements of the usage pattern that are not explicitly outlined. The documentation should clearly demonstrate:

• How to use app context in a Celery task
• How to unit test context-dependent Celery tasks

Even as someone with reasonable experience in Python and some with Flask, I had a hard time finding the answer to this is the docs.

TODO:

• [ ] Review contributing guidelines
• [ ] Prepare updated example code at examples/celery
• [ ] Draft docs update at docs/patterns/celery.rst

[michaelconan]

Bad example: Medium

Creates a separate app instance rather than using the flask.current_app function.

[michaelconan]

Use conftest.py for shared fixtures - take example from tutorial.

Given extent of tutorial example may not need to worry about tests here... or just celery task test.