Add newsletter dropdown caret hover

[janbrasna]

One-line summary

Fixes select dropdown affordance blending in with background when active.

Significant changes and points to review

1. This refactors the caret to use m24-small icons.
2. (Also cleans up any old assets no longer used.)
3. Adds RTL support.
4. Adds hover state.
5. Improves alignment for more type/size resilience.

Issue / Bugzilla link

Fixes #15716

Testing

http://localhost:8000/en-US/#colophon

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.06%. Comparing base (f47a17d) to head (a03eefa).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15719   +/-   ##
=======================================
  Coverage   79.06%   79.06%           
=======================================
  Files         160      160           
  Lines        8355     8355           
=======================================
  Hits         6606     6606           
  Misses       1749     1749           

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[wen-2018]

Looks good! r+