Replace loading state with an icon, show dashboard skeleton earlier

What does this MR do?

Part of #217758 (closed). Replaces the loading state with an icon and allows dashboard elements to be shown earlier.

1. Does away with the loading global state in favor of a big loading icon.
2. Shows the dashboard "structure" as soon as it is loaded, so collapsible groups appear early but collapsed

ToDo: Check on the kubernetes health dashboard. Checked!

Screenshots

Before

Previously we waited for any response from Prometheus to be loaded, so the empty state was displayed for longer.

1. loading dashboard (fast)	2. loading data (slow)	3. first data loaded (fast)	3. all data loaded

After

Now we should the dashboard as soon as the skeleton is loaded, but the empty charts appear hidden, until some data is loaded.

1. loading dashboard (fast)	2. loading data (slow)	3. first data loaded (fast)	3. all data loaded

Screencast (a bit slow, try to watch until the end)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team