Remove user context URL params

[mwtrew]

Stops checking for stage and examBoard URL parameters when setting the user context, and stops syncing the current user context with the URL parameters.

Also refactors the user context derivation logic into a unit-testable function, determineUserContext(), and adds unit tests.

[codecov]

Codecov Report

Attention: Patch coverage is 87.80488% with 5 lines in your changes missing coverage. Please review.

Project coverage is 36.56%. Comparing base (3183eb7) to head (7e3dfe3).
Report is 140 commits behind head on master.

Files with missing lines	Patch %	Lines
...p/components/elements/inputs/UserContextPicker.tsx	0.00%	4 Missing ⚠️
src/app/services/userViewingContext.ts	97.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1195      +/-   ##
==========================================
+ Coverage   36.46%   36.56%   +0.10%     
==========================================
  Files         435      435              
  Lines       19289    19285       -4     
  Branches     5691     5686       -5     
==========================================
+ Hits         7033     7052      +19     
+ Misses      12218    12195      -23     
  Partials       38       38              

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[mlt47]

This looks good.
Glad that there are tests to describe the expected behaviour more declaratively.
#1204 should also be merged for this next release to minimise the downsides of removing this query param syncing feature.