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/selectors/index.js

@@ -76,7 +76,7 @@ export const makeGetNotification = () => {
 };
 
 export const getAccountGallery = createSelector([
-  (state, id) => state.getIn(['timelines', 'accounts_media_timelines', id, 'items'], Immutable.List()),
+  (state, id) => state.getIn(['timelines', `account:${id}:media`, 'items'], Immutable.List()),
   state       => state.get('statuses'),
 ], (statusIds, statuses) => {
   let medias = Immutable.List();