Add ability to follow hashtags in web UI (#18862)

app/javascript/mastodon/reducers/index.js

@@ -39,6 +39,7 @@ import markers from './markers';
 import picture_in_picture from './picture_in_picture';
 import accounts_map from './accounts_map';
 import history from './history';
+import tags from './tags';
 
 const reducers = {
   announcements,
@@ -81,6 +82,7 @@ const reducers = {
   markers,
   picture_in_picture,
   history,
+  tags,
 };
 
 export default combineReducers(reducers);