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

devenv/dev-dashboards-without-uid/panel_tests_graph.json

@@ -1632,10 +1632,7 @@
   "revision": 8,
   "schemaVersion": 16,
   "style": "dark",
-  "tags": [
-    "gdev",
-    "panel-tests"
-  ],
+  "tags": ["gdev", "panel-tests"],
   "templating": {
     "list": []
   },
@@ -1644,29 +1641,8 @@
     "to": "now"
   },
   "timepicker": {
-    "refresh_intervals": [
-      "5s",
-      "10s",
-      "30s",
-      "1m",
-      "5m",
-      "15m",
-      "30m",
-      "1h",
-      "2h",
-      "1d"
-    ],
-    "time_options": [
-      "5m",
-      "15m",
-      "1h",
-      "6h",
-      "12h",
-      "24h",
-      "2d",
-      "7d",
-      "30d"
-    ]
+    "refresh_intervals": ["5s", "10s", "30s", "1m", "5m", "15m", "30m", "1h", "2h", "1d"],
+    "time_options": ["5m", "15m", "1h", "6h", "12h", "24h", "2d", "7d", "30d"]
   },
   "timezone": "browser",
   "title": "Panel Tests - Graph",