Add ability to follow hashtags (#18809)

config/routes.rb

@@ -530,6 +530,15 @@ Rails.application.routes.draw do
         resource :note, only: :create, controller: 'accounts/notes'
       end
 
+      resources :tags, only: [:show], constraints: { id: /#{Tag::HASHTAG_NAME_RE}/ } do
+        member do
+          post :follow
+          post :unfollow
+        end
+      end
+
+      resources :followed_tags, only: [:index]
+
       resources :lists, only: [:index, :create, :show, :update, :destroy] do
         resource :accounts, only: [:show, :create, :destroy], controller: 'lists/accounts'
       end