Cleanup various incongruities and outdated elements in icon folders

[sjd210]

This seems like a lot at first glance with 120 changed files, but this is generally relatively straightforward. The changes are mostly to bring our icon folders up-to-date, within the following four categories:

1. Delete several icons that are no longer used and we have no reason to use in the future (e.g. mostly cases of non-generic icons used on Isaac Physics before the redesign like icon-books.png and question_types_sprite.svg, but also specific cases like chevron-up.svg which we use a rotated chevron-right.svg for now)
2. Groups the hex buttons used in inequality/graph sketcher together in a folder and renames the icons to reflect their use in both inequality and the graph sketcher (e.g. inequality-confirm-phy.svg -> hex-confirm-sci.svg)
3. Removes outdated icon-related css that is never used - and relatedly, removes all Sci references from RelatedContent.tsx since we now have a separate file for it in that app
4. Standardises all Ada icons in new format to 16x16px. Convert any cases that rely on the old size to use the standard icon format (e.g. "icon icon-sm icon-search" instead of "search-toggler-icon" with its own styling)

[codecov]

Codecov Report

❌ Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.55%. Comparing base (392cf27) to head (7b476a5).
⚠️ Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
src/app/components/elements/RelatedContent.tsx	0.00%	8 Missing ⚠️
.../components/elements/panels/TeacherConnections.tsx	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2083      +/-   ##
==========================================
+ Coverage   43.51%   43.55%   +0.03%     
==========================================
  Files         582      582              
  Lines       24691    24664      -27     
  Branches     8207     8164      -43     
==========================================
- Hits        10744    10742       -2     
+ Misses      13890    13865      -25     
  Partials       57       57              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.