Enable admin editing of channels in the UI.

[rtibbles]

Summary

Description of the change(s) you made

• Adds isAdmin getter to session vuex module
• Uses isAdmin getter to allow editing and managing of any channel, regardless of whether a user is a direct admin

Manual verification steps performed

1. Create a new super user with ./contentcuration/manage.py createsuperuser
2. Activate the super user
3. Login
4. Navigate to any channel
5. Confirm that the UI now lets you edit

References

Fixes#3015

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 #3051 (4a1aca9) into hotfixes (28587f8) will increase coverage by 5.13%.
The diff coverage is 93.05%.

@@ Coverage Diff @@## hotfixes #3051 +/- ##
============================================
+ Coverage 80.80% 85.93% +5.13% 
============================================
Files 281 304 +23 Lines 12659 16403 +3744 ============================================
+ Hits 10229 14096 +3867 + Misses 2430 2307 -123 

Impacted Files	Coverage Δ
contentcuration/contentcuration/decorators.py	56.60% <50.00%> (-37.60%)	⬇️
...ntentcuration/contentcuration/db/models/manager.py	91.05% <90.83%> (-8.95%)	⬇️
contentcuration/contentcuration/db/models/query.py	94.23% <93.75%> (-5.77%)	⬇️
contentcuration/contentcuration/forms.py	82.35% <94.04%> (+33.89%)	⬆️
contentcuration/contentcuration/api.py	92.06% <100.00%> (+1.43%)	⬆️
contentcuration/contentcuration/apps.py	100.00% <100.00%> (+11.11%)	⬆️
contentcuration/contentcuration/celery.py	90.00% <100.00%> (-1.67%)	⬇️
...tentcuration/contentcuration/context_processors.py	100.00% <100.00%> (ø)
...ontentcuration/contentcuration/db/advisory_lock.py	100.00% <100.00%> (ø)
...tcuration/contentcuration/db/models/expressions.py	95.23% <100.00%> (-4.77%)	⬇️
... and 190 more

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[bjester]

Looks good