Fix performance of home feed regeneration (#12084)

Fetching statuses from all followed accounts at once takes too long
within Postgres. Fetching them one by one and merging in Ruby
could be a lot less resource-intensive

Because the query for dynamically fetching the home timeline is so
heavy, we can no longer offer it when the home timeline is missing

app/controllers/api/v1/timelines/home_controller.rb

@@ -13,7 +13,7 @@ class Api::V1::Timelines::HomeController < Api::BaseController
     render json: @statuses,
            each_serializer: REST::StatusSerializer,
            relationships: StatusRelationshipsPresenter.new(@statuses, current_user&.account_id),
-           status: regeneration_in_progress? ? 206 : 200
+           status: account_home_feed.regenerating? ? 206 : 200
   end
 
   private
@@ -62,8 +62,4 @@ class Api::V1::Timelines::HomeController < Api::BaseController
   def pagination_since_id
     @statuses.first.id
   end
-
-  def regeneration_in_progress?
-    Redis.current.exists("account:#{current_account.id}:regeneration")
-  end
 end