User context improvements

[mwtrew]

Various updates to the context picker and viewing contexts:

• Remove now-inapplicable "unusual context" logic - always show the tooltip with an explanation for the current viewing context.
• Fix problems with "no content on this page" message not showing on Topic pages. This now relies on the audience field in the topic summary content object being correct, as with Concept pages.
• Add a reset button to the context picker, which clears the transient context if set.

Best reviewed with whitespace changes hidden.

[codecov]

Codecov Report

Attention: Patch coverage is 26.86567% with 49 lines in your changes missing coverage. Please review.

Project coverage is 36.64%. Comparing base (a4f1a00) to head (9348394).
Report is 14 commits behind head on master.

Files with missing lines	Patch %	Lines
...p/components/elements/inputs/UserContextPicker.tsx	4.44%	42 Missing and 1 partial ⚠️
...nents/navigation/IntendedAudienceWarningBanner.tsx	33.33%	2 Missing ⚠️
src/app/state/slices/internalAppState.ts	0.00%	2 Missing ⚠️
src/app/components/pages/Topic.tsx	0.00%	1 Missing ⚠️
src/app/services/userViewingContext.ts	93.75%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1242      +/-   ##
==========================================
+ Coverage   36.62%   36.64%   +0.02%     
==========================================
  Files         451      451              
  Lines       19763    19763              
  Branches     6478     6475       -3     
==========================================
+ Hits         7238     7243       +5     
+ Misses      11880    11875       -5     
  Partials      645      645              

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