fix: don't show groups cards zero state while groups data is loading

[marlonkeating]

Jira Ticket

This change adds a loading skeleton for the groups cards section while groups are loading.

Testing Instructions

• Check out branch and start with npm run start:stage
• Navigate to https://localhost.stage.edx.org:1991/exec-ed-2u-integration-qa/admin/people-management
• Verify groups cards section has a loading Skeleton, which is replaced by the groups cards when loading finishes

For all changes

• Ensure adequate tests are in place (or reviewed existing tests cover changes)

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 85.80%. Comparing base (825bc69) to head (f66821e).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1380   +/-   ##
=======================================
  Coverage   85.80%   85.80%           
=======================================
  Files         625      625           
  Lines       13639    13642    +3     
  Branches     2814     2858   +44     
=======================================
+ Hits        11703    11706    +3     
  Misses       1866     1866           
  Partials       70       70           

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