Show loading spinner for security dashboard charts

What does this MR do?

On the project, group, and instance-level security dashboards, we show a sparkline chart and an accordion for the project security status:

While data is loading, the chart and accordion shows no 0 vulnerabilities, which is misleading until the data finally loads. This can be especially misleading if the data takes a long time to load, like it does for the gitlab-org group:

https://gitlab.com/groups/gitlab-org/-/security/dashboard

This MR fixes it by adding a loading spinner while the data is loading (note that there's also a full-page loading spinner that shows before the two panels are shown):

Before	After

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #284471 (closed)