Replace sample Celery result_backend in config

[davelopez]

Closes #17881
Fixes #17878

It just replaces the default Celery result_backend from redis to an SQLite database so it should "work out of the box" with a development instance. Production instances should change to something more robust as hinted in the documentation.

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[bernt-matthias]

All I can add here is that I used sqlite on my test instance and it worked.

[jdavcs]

This looks good. Thank you for all the edits!

Regenerate the config, and it's good to go.

[davelopez]

Test failures unrelated :)

[bernt-matthias]

Thanks a lot @davelopez for adding docs and all this for celery.