Staging tree fixes

[jayoshih]

• Fixed issue where adding /staging to the end of any channel url ended up throwing an error instead of showing no changes to review page
• Fixed node diff endpoint permission checks for non-admins

[kollivier]

LGTM!

[codecov]

Codecov Report

Merging #2835 (bad6b59) into hotfixes (28587f8) will increase coverage by 4.05%.
The diff coverage is 88.73%.

@@ Coverage Diff @@## hotfixes #2835 +/- ##
============================================
+ Coverage 80.80% 84.85% +4.05% 
============================================
Files 281 292 +11 Lines 12659 15338 +2679 ============================================
+ Hits 10229 13015 +2786 + Misses 2430 2323 -107 

Impacted Files	Coverage Δ
contentcuration/contentcuration/decorators.py	56.60% <50.00%> (-37.60%)	⬇️
contentcuration/contentcuration/forms.py	69.06% <72.41%> (+20.61%)	⬆️
...ntentcuration/contentcuration/db/models/manager.py	93.49% <93.33%> (-6.51%)	⬇️
contentcuration/contentcuration/api.py	92.06% <100.00%> (+1.43%)	⬆️
...tentcuration/contentcuration/context_processors.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%)	⬇️
contentcuration/contentcuration/utils/cache.py	31.70% <0.00%> (-31.93%)	⬇️
... and 173 more

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