Fix #3665 - Refactor timelines reducer (#3686)

* Move ancestors/descendants out of timelines reducer

* Refactor timelines reducer

All types of timelines now have a flat structure and use the same
reducer functions and actions

* Reintroduce some missing behaviours

* Fix wrong import in reports

* Fix includes typo

* Fix issue related to "next" pagination in timelines and notifications

* Fix bug with timeline's initial state, expandNotifications

app/javascript/mastodon/reducers/index.js

@@ -17,6 +17,7 @@ import settings from './settings';
 import status_lists from './status_lists';
 import cards from './cards';
 import reports from './reports';
+import contexts from './contexts';
 
 export default combineReducers({
   timelines,
@@ -37,4 +38,5 @@ export default combineReducers({
   settings,
   cards,
   reports,
+  contexts,
 });