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/features/ui/index.js

@@ -42,6 +42,7 @@ import {
   FollowRequests,
   FavouritedStatuses,
   BookmarkedStatuses,
+  FollowedTags,
   ListTimeline,
   Blocks,
   DomainBlocks,
@@ -216,6 +217,7 @@ class SwitchingColumnsArea extends React.PureComponent {
           <WrappedRoute path='/follow_requests' component={FollowRequests} content={children} />
           <WrappedRoute path='/blocks' component={Blocks} content={children} />
           <WrappedRoute path='/domain_blocks' component={DomainBlocks} content={children} />
+          <WrappedRoute path='/followed_tags' component={FollowedTags} content={children} />
           <WrappedRoute path='/mutes' component={Mutes} content={children} />
           <WrappedRoute path='/lists' component={Lists} content={children} />