Support mode_studied being an array of multiple modes

[JGreenlee]

mode_studied is currently a string, but we want to support multiple modes of interest. We can allow it to be string | string[] and handle both cases at runtime

Updated app config typing accordingly

[JGreenlee]

I tested this by locally hosting a modified stage-program config to have ["e-bike", "bike"]

---

Then I signed in on the Simulator with one of my real opcodes and added a few fake mode labels.
The walk trip does not prompt for replaced mode, but the bike and e-bike trips do

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 29.78%. Comparing base (39b2e2e) to head (078037a).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
www/js/survey/multilabel/confirmHelper.ts	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1201      +/-   ##
==========================================
+ Coverage   29.76%   29.78%   +0.02%     
==========================================
  Files         123      123              
  Lines        4959     4962       +3     
  Branches     1096     1142      +46     
==========================================
+ Hits         1476     1478       +2     
+ Misses       3483     3482       -1     
- Partials        0        2       +2     

Flag	Coverage Δ
unit	29.78% <80.00%> (+0.02%)	⬆️

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Files with missing lines	Coverage Δ
www/js/survey/multilabel/confirmHelper.ts	87.25% <80.00%> (-0.63%)	⬇️

... and 2 files with indirect coverage changes

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[shankari]

we can remove this code once all the existing programs with a single mode of interest have been archived, right?

Can we add a TODO: here to flag that?