Add trends UI with admin and user settings (#11502)

app/javascript/mastodon/reducers/trends.js

@@ -0,0 +1,23 @@
+import { TRENDS_FETCH_REQUEST, TRENDS_FETCH_SUCCESS, TRENDS_FETCH_FAIL } from '../actions/trends';
+import { Map as ImmutableMap, List as ImmutableList, fromJS } from 'immutable';
+
+const initialState = ImmutableMap({
+  items: ImmutableList(),
+  isLoading: false,
+});
+
+export default function trendsReducer(state = initialState, action) {
+  switch(action.type) {
+  case TRENDS_FETCH_REQUEST:
+    return state.set('isLoading', true);
+  case TRENDS_FETCH_SUCCESS:
+    return state.withMutations(map => {
+      map.set('items', fromJS(action.trends));
+      map.set('isLoading', false);
+    });
+  case TRENDS_FETCH_FAIL:
+    return state.set('isLoading', false);
+  default:
+    return state;
+  }
+};