Templating: removes old Angular variable system and featureToggle (#24779)

* Chore: initial commit

* Tests: fixes MetricsQueryEditor.test.tsx

* Tests: fixes cloudwatch/specs/datasource.test.ts

* Tests: fixes stackdriver/specs/datasource.test.ts

* Tests: remove refrences to CustomVariable

* Refactor: moves DefaultVariableQueryEditor

* Refactor: moves utils

* Refactor: moves types

* Refactor: removes variableSrv

* Refactor: removes feature toggle newVariables

* Refactor: removes valueSelectDropDown

* Chore: removes GeneralTabCtrl

* Chore: migrates RowOptions

* Refactor: adds RowOptionsButton

* Refactor: makes the interface more explicit

* Refactor: small changes

* Refactor: changed type as it can be any variable type

* Tests: fixes broken test

* Refactor: changes after PR comments

* Refactor: adds loading state and call to onChange in componentDidMount

packages/grafana-e2e/cypress/fixtures/example.json

@@ -0,0 +1,5 @@
+{
+  "name": "Using fixtures to represent data",
+  "email": "hello@cypress.io",
+  "body": "Fixtures are a great way to mock data for responses to routes"
+}