DEV: improves sk api (#9653)

- reduces the API to 3 actions for now: appendContent/prependContent/onChange
- well tested
- removes all previous APIS which were only half supported or too dangerous as they could collide with other plugins or core behaviors
- this plugins also puts every sk test helpers in one file

test/javascripts/components/select-kit/notifications-button-test.js

@@ -1,5 +1,8 @@
 import componentTest from "helpers/component-test";
-import { testSelectKitModule, setDefaultState } from "./select-kit-test-helper";
+import {
+  testSelectKitModule,
+  setDefaultState
+} from "helpers/select-kit-helper";
 
 testSelectKitModule("notifications-button");