feat: implement the program nudge suggested course URL override

[pwnage101]

Implement platform's OVERRIDE_PROGRAM_NUDGE_SUGGESTED_COURSE_URL hook with the following goal: learners of a customer with the learner portal enabled should be sent to the portal's course page.

ENT-12352

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Related PRs:

• feat: implement the program nudge suggested course URL override #2696
• feat: add OVERRIDE_PROGRAM_NUDGE_SUGGESTED_COURSE_URL pluggable override edx/edx-platform#485

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.06%. Comparing base (3a0f02e) to head (20ba001).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2696      +/-   ##
==========================================
+ Coverage   87.05%   87.06%   +0.01%     
==========================================
  Files         264      265       +1     
  Lines       17278    17292      +14     
  Branches     1707     1708       +1     
==========================================
+ Hits        15041    15055      +14     
  Misses       1898     1898              
  Partials      339      339              

Flag	Coverage Δ
unittests	87.06% <100.00%> (+0.01%)	⬆️

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