Upgrade celery and use database backend for task results

[bjester]

Summary

Description of the change(s) you made

• Updates to the latest version of celery available for Python 3.6
• Replaces Studio's own Task model and uses TaskResult from django-celery-results, which means this switches the celery result backend from redis to postgres
• Removes eager processing (synchronous) of tasks within python tests, and explicitly enables it when needed
• Refactors testing of asynchronous tasks to be entirely contained within test files-- previously, test tasks lived within the core codebase
• Replaces string mapping of tasks and related queuing functions with task instance methods
• Re-enables late acknowledgement of tasks, superseding Configure task behavior to acknowledge after execution to trigger requeuing when interrupted #3287

Manual verification steps performed

• General channel editing features such as uploading files, creating topics, and importing resouces
• Channel publishing

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Reviewer guidance

How can a reviewer test these changes?

Are there any risky areas that deserve extra testing?

• Task related behaviors which now includes most channel editing features

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Contributor's Checklist

PR process:

• If this is an important user-facing change, PR or related issue the CHANGELOG label been added to this PR. Note: items with this label will be added to the CHANGELOG at a later time
• If this includes an internal dependency change, a link to the diff is provided
• The docs label has been added if this introduces a change that needs to be updated in the user docs?
• If any Python requirements have changed, the updated requirements.txt files also included in this PR
• Opportunities for using Google Analytics here are noted
• Migrations are safe for a large db

Studio-specifc:

• All user-facing strings are translated properly
• The notranslate class been added to elements that shouldn't be translated by Google Chrome's automatic translation feature (e.g. icons, user-generated text)
• All UI components are LTR and RTL compliant
• Views are organized into pages, components, and layouts directories as described in the docs
• Users' storage used is recalculated properly on any changes to main tree files
• If there new ways this uses user data that needs to be factored into our Privacy Policy, it has been noted.

Testing:

• Code is clean and well-commented
• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Any new interactions have been added to the QA Sheet
• Critical and brittle code paths are covered by unit tests

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Reviewer's Checklist

This section is for reviewers to fill out.

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[rtibbles]

Looking good - one thing that seems like it might be a blocker from the read through, another that could do with some cleanup.

[rtibbles]

Looks like we need to remove the celery:dashboard command from package.json and from the yarn run services invocation, because it just errors now. I don't know that anyone was using it, so I don't think we need to worry about fixing it.

[rtibbles]

Comments addressed and manual testing checks out. Just cleaning up the celery dashboard seems like the last TODO!

[rtibbles]

I pinged and it ponged.