Fix "randomize question order" field not being synced when toggling its checkbox in EditModal

[jonboiser]

Description

1. Adds randomize to ExtraFieldsSerializer
2. Fixes diffing code to properly handle the case when going from randomized = true -> randomized = false

Issue Addressed (if applicable)

Fixes #2864

Steps to Test

• Edit an exercise. All the fields should synchronize as they are modified

[rtibbles]

Code changes look good - a regression test would be great.

[rtibbles]

Could you either add this to this test here: https://github.com/learningequality/studio/blob/hotfixes/contentcuration/contentcuration/tests/viewsets/test_contentnode.py#L534 or create a similar test specifically for randomize?

[jonboiser]

I added a section for randomize in th existing unit test

[codecov]

Codecov Report

Merging #2956 (1a66bf3) into hotfixes (28587f8) will increase coverage by 4.78%.
The diff coverage is 93.00%.

@@             Coverage Diff              @@
##           hotfixes    #2956      +/-   ##
============================================
+ Coverage     80.80%   85.59%   +4.78%     
============================================
  Files           281      299      +18     
  Lines         12659    15893    +3234     
============================================
+ Hits          10229    13603    +3374     
+ Misses         2430     2290     -140     

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

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9edb358...27afd30. Read the comment docs.