add a loading state to DashboardCard

[gumaerc]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/7106

Description (What does it do?)

This PR adds a loading state to the DashboardCard component, currently used in the enrollments dashboard. As part of this, some minor refactoring was done in the structure of DashboardCard.

Screenshots (if appropriate):

How can this be tested?

• On line 199 of frontends/main/src/app-pages/DashboardPage/CoursewareDisplay/EnrollmentDisplay.tsx, temporarily set isLoading={true}, like this:

      <EnrollmentExpandCollapse
        shownEnrollments={shownEnrollments}
        hiddenEnrollments={expired}
        isLoading={true}
      />

• Spin up mit-learn on this branch
• Ensure you have Posthog connected to your local instance of mit-learn
• In your connected Posthog project, make sure you have added and enabled the enrollment-dashboard feature flag
• Visit http://open.odl.local:8062/dashboard and log in (if you are not already logged in)
• Ensure that you see the loading state in both desktop and mobile views
• Set the line we changed earlier back to isLoading={isLoading} and save the file
• Ensure that the cards look unchanged compared to RC

[github-actions]

OpenAPI Changes

Show/hide No detectable change.

[shanbady]

Looks good LGTM 👍