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

public/app/features/dashboard/services/ChangeTracker.ts

@@ -134,7 +134,7 @@ export class ChangeTracker {
     });
 
     // ignore template variable values
-    _.each(dash.getVariables(), variable => {
+    _.each(dash.getVariables(), (variable: any) => {
       variable.current = null;
       variable.options = null;
       variable.filters = null;