fix: ensure consistency around start dates on course run cards in Course page

[adamstankiewicz]

Adds parity with start-based assignments by updating business logic around which start is displayed on the available course run cards in the Course page route.

For all changes

• Ensure adequate tests are in place (or reviewed existing tests cover changes)
• Ensure English strings are marked for translation. See documentation for more details.

Only if submitting a visual change

• Ensure to attach screenshots
• Ensure to have UX team confirm screenshots

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.45%. Comparing base (ac21d02) to head (18f724e).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1203   +/-   ##
=======================================
  Coverage   88.45%   88.45%           
=======================================
  Files         398      398           
  Lines        8459     8459           
  Branches     2072     2073    +1     
=======================================
  Hits         7482     7482           
  Misses        934      934           
  Partials       43       43           

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