Scopes: Improve suggested dashboards listing

[bfmatei]

What is this feature?

This PR brings improvements to how and when the dashboards list is loaded.

Besides the improvements, this PR also fixes an issue where unselecting a scope would leave the suggested dashboards list intact.

Why do we need this feature?

So, the changes here are needed for various reasons. I'll take each change one by one and explain the rationale behind it.

• only load suggested dashboards list when the scope selector is expanded
  • there is no need to load the dashboards list if we don't present it to the user
  • this will avoid a lot of API unnecessary calls as well as saving a lot of bandwidth
• keep a map of loaded dashboards
  • because a dashboard can be suggested by multiple scopes, we only need to load its details once
  • this won't save a ton of API unnecessary calls but it will save a lot of bandwidth as we were loading the entire dashboard JSON every time
• keep a map of dashboard IDs for each scope
  • because a scope can be toggled off and then on again, we will only load the dashboards for it once
  • this has the potential to avoid unnecessary API calls

Who is this feature for?

Which issue(s) does this PR fix?:

Special notes for your reviewer:

Also a video to illustrate all the changes:

Screen.Recording.2024-05-16.at.13.29.01.mov

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[bfmatei]

Closing in favor of #88026