Add inline multiple choice questions

[jacbn]

Adds a new type of inline question, multiple choice, that creates a dropdown similar to (but slightly distinct from in structure) cloze questions in mobile view.

Also adds some type checking to InlineEntryZone such that adding further new inline questions is even easier, and collates all border / shadow colouring into the same place.

[codecov]

Codecov Report

Attention: Patch coverage is 26.47059% with 25 lines in your changes missing coverage. Please review.

Project coverage is 36.62%. Comparing base (834366f) to head (21eb979).
Report is 14 commits behind head on master.

Files with missing lines	Patch %	Lines
...nts/elements/inputs/InlineMultiChoiceEntryZone.tsx	31.81%	15 Missing ⚠️
...onents/elements/markup/portals/InlineEntryZone.tsx	16.66%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1232      +/-   ##
==========================================
+ Coverage   36.60%   36.62%   +0.02%     
==========================================
  Files         443      444       +1     
  Lines       19509    19539      +30     
  Branches     5757     5764       +7     
==========================================
+ Hits         7141     7157      +16     
- Misses      12330    12344      +14     
  Partials       38       38              

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