fix: only show course blocks in the search modal

[rpenido]

Description

This PR limits the scope of the search in the modal to only course blocks.

Testing instructions

• Using master go to the search modal and make sure you have any library XBlock indexed.
• Check out this branch, go to the search modal again, and the libraries blocks should be filtered out.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.68%. Comparing base (09822c2) to head (7136ad1).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1148   +/-   ##
=======================================
  Coverage   92.68%   92.68%           
=======================================
  Files         694      694           
  Lines       12359    12359           
  Branches     2701     2701           
=======================================
  Hits        11455    11455           
  Misses        873      873           
  Partials       31       31           

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[bradenmacdonald]

Thanks! I tested and it seems to be working well.

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