Show suggested follows on search screen in mobile layout (#9010)

Reminder: Suggestions were added in #7918 and are based on who you
interact with who you do not follow. E.g. if you boost someone a lot
from seeing other people's boosts of that person, it makes sense you
might be interested in following the original source; or if you reply
to someone a lot, maybe you'd want to follow them

Each suggestion can be dismissed

app/javascript/mastodon/reducers/suggestions.js

@@ -0,0 +1,30 @@
+import {
+  SUGGESTIONS_FETCH_REQUEST,
+  SUGGESTIONS_FETCH_SUCCESS,
+  SUGGESTIONS_FETCH_FAIL,
+  SUGGESTIONS_DISMISS,
+} from '../actions/suggestions';
+import { Map as ImmutableMap, List as ImmutableList, fromJS } from 'immutable';
+
+const initialState = ImmutableMap({
+  items: ImmutableList(),
+  isLoading: false,
+});
+
+export default function suggestionsReducer(state = initialState, action) {
+  switch(action.type) {
+  case SUGGESTIONS_FETCH_REQUEST:
+    return state.set('isLoading', true);
+  case SUGGESTIONS_FETCH_SUCCESS:
+    return state.withMutations(map => {
+      map.set('items', fromJS(action.accounts.map(x => x.id)));
+      map.set('isLoading', false);
+    });
+  case SUGGESTIONS_FETCH_FAIL:
+    return state.set('isLoading', false);
+  case SUGGESTIONS_DISMISS:
+    return state.update('items', list => list.filterNot(id => id === action.id));
+  default:
+    return state;
+  }
+};