Fix pb-message boilerplate templates ignored by generic container

[salman2013]

Summary

Bug:
When inside a Problem Builder (or other StudioContainerWithNestedXBlocksMixin) container block, the "Add Component" buttons for pb-message variants (Message complete / incomplete / Max attempts) all create the same default block, ignoring the selected boilerplate template.
For detail look at this comment: #3058 (comment)

Ticket: #3156

What we fixed in this PR:

• Buttons for blocks declared via StudioContainerWithNestedXBlocksMixin (e.g. pb-message complete/incomplete/max-attempts) were ignoring the boilerplate_name from their template spec, so every variant always created the default block.
• Fixed by passing firstTemplate?.boilerplateName from the button onClick through to the handleCreateNewCourseXBlock API call as boilerplate.

Test Results:

[openedx-webhooks]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.79%. Comparing base (211a16c) to head (a03122c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3157   +/-   ##
=======================================
  Coverage   95.79%   95.79%           
=======================================
  Files        1398     1398           
  Lines       33473    33473           
  Branches     7886     7875   -11     
=======================================
  Hits        32064    32064           
  Misses       1350     1350           
  Partials       59       59           

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