Subject Overview Page cleanup

[sjd210]

General changes to line things up with the mock ups for each Subject Overview Page:

• Adding symbols to explain practice and challenge difficulties
• Including a greyed out option for GCSE Biology in the card list
  • (+ removing blanks from the card list on small screens)
• Adding the example question deck to the bottom of the screen.
  • Physics hasn't made one yet. We can confirm it has the same id as the commented one and add it in fairly easily once available.

[codecov]

Codecov Report

Attention: Patch coverage is 53.84615% with 12 lines in your changes missing coverage. Please review.

Project coverage is 40.54%. Comparing base (4afc7c1) to head (a900e72).
Report is 2 commits behind head on redesign-2024.

Files with missing lines	Patch %	Lines
src/app/components/pages/SubjectOverviewPage.tsx	38.46%	8 Missing ⚠️
...rc/app/components/elements/svg/DifficultyIcons.tsx	81.81%	2 Missing ⚠️
...ents/elements/list-groups/AbstractListViewItem.tsx	0.00%	1 Missing ⚠️
src/app/components/pages/SubjectLandingPage.tsx	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                @@
##           redesign-2024    #1475      +/-   ##
=================================================
- Coverage          40.60%   40.54%   -0.06%     
=================================================
  Files                495      495              
  Lines              22087    22101      +14     
  Branches            6531     7305     +774     
=================================================
- Hits                8968     8961       -7     
+ Misses             13082    12515     -567     
- Partials              37      625     +588     

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