Catch duplicate editor assignment for newly created channels.

[rtibbles]

Summary

Description of the change(s) you made

• Catches integrity errors when assigning an editor to a newly created channel
• Assuming this happens if the same change gets applied twice

Manual verification steps performed

All tests still pass

References

Fixes #3049

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

[codecov]

Codecov Report

Merging #3099 (40d6466) into unstable (3da1570) will increase coverage by 0.03%.
The diff coverage is 95.12%.

@@             Coverage Diff              @@
##           unstable    #3099      +/-   ##
============================================
+ Coverage     85.71%   85.75%   +0.03%     
============================================
  Files           298      298              
  Lines         15879    15912      +33     
============================================
+ Hits          13611    13645      +34     
+ Misses         2268     2267       -1     

Impacted Files	Coverage Δ
...ontentcuration/contentcuration/viewsets/channel.py	79.31% <60.00%> (-0.44%)	⬇️
...contentcuration/tests/views/test_views_internal.py	97.44% <100.00%> (+0.20%)	⬆️
contentcuration/contentcuration/views/internal.py	85.75% <100.00%> (+0.37%)	⬆️
contentcuration/contentcuration/tasks.py	63.25% <0.00%> (+0.53%)	⬆️
...uration/contentcuration/tests/test_urlendpoints.py	95.83% <0.00%> (+2.08%)	⬆️

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[pcenov]

Tested at https://hotfixes.studio.learningequality.org/ that I cannot invite the same email twice.