Add loading state to customizable dashboards

What does this MR do and why?

Related to #378277 (closed)

Renders a GlLoadingIcon while the selected dashboard is being loaded.

Screenshots or screen recordings

With artificial 3s delay

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With no delay on local. Notice it doesn't show the loader the 2nd time a dashboard is opened

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How to set up and validate locally

1. Make sure you are on GitLab Ultimate.
2. Enable the product_analytics_internal_preview feature flag

   Feature.enable(:product_analytics_internal_preview)

3. Visit any PA project pages such as http://127.0.0.1:3000/flightjs/Flight/-/product_analytics/dashboards/
4. Open any of the listed dashboards.
5. The loader should appear before the data is loaded. You might want to throttle your Network.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.