Add is_public to APIProblemDetail

[jtyliu]

Adds is_public to tell which problems are public, so you don't leak your private problems when using the api token

[codecov-io]

Codecov Report

Merging #1564 into master will decrease coverage by 0.58%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1564      +/-   ##
==========================================
- Coverage   46.15%   45.57%   -0.59%     
==========================================
  Files         211      234      +23     
  Lines       12087    13038     +951     
==========================================
+ Hits         5579     5942     +363     
- Misses       6508     7096     +588     

Impacted Files	Coverage Δ
judge/views/api/api_v2.py	49.01% <ø> (ø)
src/martor/martor/widgets.py	53.84% <0.00%> (ø)
src/martor/martor/models.py	57.14% <0.00%> (ø)
src/ansi2html/ansi2html/style.py	18.33% <0.00%> (ø)
...-wpadmin/wpadmin/templatetags/wpadmin_menu_tags.py	30.33% <0.00%> (ø)
src/dmoj-wpadmin/wpadmin/__init__.py	100.00% <0.00%> (ø)
src/django-sortedm2m/sortedm2m/fields.py	85.50% <0.00%> (ø)
src/martor/martor/views.py	38.23% <0.00%> (ø)
src/django-sortedm2m/sortedm2m/forms.py	25.35% <0.00%> (ø)
dmoj/local_settings.py	0.00% <0.00%> (ø)
... and 14 more

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[Ninjaclasher]

Please also submit a PR with the doc changes.

[jtyliu]

Available here.

[Xyene]

Thanks!