Fix activate channel endpoint behaviour

[rtibbles]

Description

• Ensures proper behaviour of the activate channel endpoint by ensuring that staged files and active files are properly compared against current usage.
• Adds tests for the activate channel endpoint
• Properly wraps the activate channel endpoint so that it gets a non-anonymous user from the request

[codecov]

Codecov Report

Merging #2918 (7fafb00) into hotfixes (28587f8) will increase coverage by 4.73%.
The diff coverage is 93.00%.

@@ Coverage Diff @@## hotfixes #2918 +/- ##
============================================
+ Coverage 80.80% 85.53% +4.73% 
============================================
Files 281 299 +18 Lines 12659 15827 +3168 ============================================
+ Hits 10229 13538 +3309 + Misses 2430 2289 -141 

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 181 more

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[kollivier]

LGTM!