Dashboard panel groups: nested dashboards via reference

[stacey-gammon]

A more thorough implementation idea to replace the less thought out #12313.

Overview

Allow users to add all the panels from one dashboard inside another dashboard as a reference. I'm calling these "dashboard panel groups" because the only part of the dashboard that should be added is the panels, and not other configuration aspects (e.g. theme).

What problems would this solve?

• Many users have multiple dashboards with the same visualizations where only the index pattern differs. This feature, in conjunction with this one: Dashboard level index patterns #16917 would solve that issue, so that only one dashboard would need to be maintained, all other dashboards would have a dashboard level index pattern and a single nested dashboard panel group. Then only one dashboard needs to be maintained and all other dashboards are automatically updated as well. This covers the idea of dashboard "templates".
• A lot of users want a way to group and collapse panels - [Dashboards] Collapsible Panels #1547. This gives us the panel grouping, then we'd just need to add the ability to collapse individual panels.
• Dashboards take a long time to load. If we add in a feature to skip the es request for collapsed panels then this gives the user more control over how long their dashboards take to load.

Implementation specifics

• All state data should be supplied by the top most dashboard, so for instance, you wouldn't see a dark themed dashboard inside a light themed one.
• We should prevent infinite loops so a dashboard can't embed itself.
• For usability I think we should also prevent embedding dashboards that already have embedded dashboards on them (so only "flat" dashboards should show up in the list). It'd just start to get ugly with all the padding.
• Also for usability, I think we should require these panels to be full width and never allow them to be smaller. This would force them to act as "rows" so if we support collapsing them (and perhaps only them) we don't have to worry about changing dashboard layouts, which is a concern if we allow collapsing every panel.

cc @alexfrancoeur @AlonaNadler

[ThomThomson]

Closing this in favour of #88710.

We are attempting to build this kind of system in a more flexible manner, that exists fully within one dashboard.