Templating: Migrates some variable types from Angular to React/Redux

[mckn]

What this PR does / why we need it:
Fourth iteration trying to move template variables to Redux. This iteration takes the React/Redux parts from the third iteration #21873 and does (almost) no integration with Angular components. Everything is behind the feature toggle called newVariables and will be a completely new property in the dashboard.json => variables.list.

Misc

• fix tests
• fix strict null errrors
• write tests
• cleanup/refactor code
• update e2e tests
• documentation
• celebrate 🎉

Known issues so far:

• focus is not following highlighted picker option when navigating with keys (same on play.grafana.com)

When we have nothing else to do:

• Simplify state handling
• Create a tag component to handle the picker tags
• Change so the VariableTag and VariableOption uses text/value: string[] instead of string | string[]
• Change so thunk signatures take uuid as parameter instead of variable to avoid stupid bugs
• Write a class that walks the variable tree and create an execution order.

Upcoming PRs

• Refactor Picker/Picker actions and state into something that can be reused between custom/query/datasource variable types
• Introduce custom variable type
• Introduce textbox variable type
• Introduce constant variable type

Which issue(s) this PR fixes:
Closes #19896

Special notes for your reviewer:
Not covered in this PR

• Template Variable save UX #20703
• New Form Styles
• opened a new Issue with the leftovers here: Template Variables move to React/Redux Leftovers #22564

[torkelo]

Ok 99/99 files viewed!

Amazing work ❤️

Was not much that I could find that looked off or I could not understand the reasons for.

Will do more testing but since it's behind a feature toggle I think we can do that after merge as well.

[hugohaggmark]

Ok 99/99 files viewed!

Amazing work ❤️

Was not much that I could find that looked off or I could not understand the reasons for.

Will do more testing but since it's behind a feature toggle I think we can do that after merge as well.

Thanks for all the great feedback @torkelo and @dprokop and it feels great to have come this far finally!