Add listing of followed hashtags (#21773)

* Add followed_tags route.

This at least gets us to the point where the page can actually be
rendered, although it doesn't display any hashtags (yet?).

Attempting to implement #20763.

* Fix minor issues.

* I've got the followed tags data partially working

But the Hashtag component errors for some reason. Something about the
value of the history attribute being invalid.

* Fix a mistake in the code

* Minor change.

* Get the followed hashtags list fully working.

Still need to add the Follow/Unfollow buttons, though.

* Resolve JS linter issues.

* Add pagination logic to followed tags list view.

However, it currently loads further pages immediately on page load, so
that's not ideal. Need to figure that one out.

* Appease the linter.

* Apply suggestions from code review

Co-authored-by: Claire <claire.github-309c@sitedethib.com>

* Fixes and resolve some other feedback.

* Use set/update instead of setIn/updateIn.

Co-authored-by: Claire <claire.github-309c@sitedethib.com>

app/javascript/mastodon/reducers/index.js

@@ -40,6 +40,7 @@ import picture_in_picture from './picture_in_picture';
 import accounts_map from './accounts_map';
 import history from './history';
 import tags from './tags';
+import followed_tags from './followed_tags';
 
 const reducers = {
   announcements,
@@ -83,6 +84,7 @@ const reducers = {
   picture_in_picture,
   history,
   tags,
+  followed_tags,
 };
 
 export default combineReducers(reducers);