Reduce server load caused by anonymous viewing. (#9059)

Do not start a session if the current user is not logged in for public-facing pages.

Mark pages that don't care about sessions as publicly cacheable.

Keep the max age as 0 so proxies and browsers will still try to retrieve an updated version but can still fall back to the stale version if the site is down or too slow.

Fixes #9035.

app/controllers/follower_accounts_controller.rb

@@ -6,6 +6,8 @@ class FollowerAccountsController < ApplicationController
   def index
     respond_to do |format|
       format.html do
+        mark_cacheable! unless user_signed_in?
+
         next if @account.user_hides_network?
 
         follows