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 85.71%. Comparing base (0716be5) to head (0012fdc).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1380   +/-   ##
=======================================
  Coverage   85.71%   85.71%           
=======================================
  Files         625      625           
  Lines       13638    13641    +3     
  Branches     2856     2812   -44     
=======================================
+ Hits        11690    11693    +3     
  Misses       1878     1878           
  Partials       70       70           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.