Updates to create channel internal endpoint to handle custom channel navigation creation

[rtibbles]

Description

Updates the create channel endpoint to allow passing the data for a custom HTML5 app to attach to the root node of a channel, and the extra fields setting that is needed to flag the root node as a custom navigation topic.

[codecov]

Codecov Report

Merging #2908 (7f5454c) into hotfixes (28587f8) will increase coverage by 4.63%.
The diff coverage is 93.00%.

@@ Coverage Diff @@## hotfixes #2908 +/- ##
============================================
+ Coverage 80.80% 85.44% +4.63% 
============================================
Files 281 298 +17 Lines 12659 15819 +3160 ============================================
+ Hits 10229 13516 +3287 + Misses 2430 2303 -127 

Impacted Files	Coverage Δ
contentcuration/contentcuration/decorators.py	56.60% <50.00%> (-37.60%)	⬇️
...ntentcuration/contentcuration/db/models/manager.py	91.20% <90.98%> (-8.80%)	⬇️
contentcuration/contentcuration/forms.py	82.85% <94.31%> (+34.40%)	⬆️
contentcuration/contentcuration/api.py	92.06% <100.00%> (+1.43%)	⬆️
...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	93.33% <100.00%> (-6.67%)	⬇️
...entcuration/contentcuration/db/models/functions.py	100.00% <100.00%> (ø)
...ation/contentcuration/tests/test_rest_framework.py	36.93% <0.00%> (-63.07%)	⬇️
contentcuration/contentcuration/utils/format.py	21.05% <0.00%> (-49.54%)	⬇️
... and 180 more

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[kollivier]

LGTM!