Fix UserTrackingConcern firing on every request, optimize some queries (#5368)

- For some reason, :if option on before_action did not work. It got
  executed every time, returned false, and the action run anyway,
  which led to the current_sign_in_at and sign_in_count being
  updated on every request
- Return "do not filter" early in FeedManager#filter_from_home? if
  the status is authored by receiver. Usually this method is not
  called for own statuses at all, but it is called when Feed#get
  uses the database
- Return early if #reload_stale_associations! has nothing to load
  to save a database query with WHERE 1=0

app/models/notification.rb

@@ -68,7 +68,10 @@ class Notification < ApplicationRecord
   class << self
     def reload_stale_associations!(cached_items)
       account_ids = cached_items.map(&:from_account_id).uniq
-      accounts    = Account.where(id: account_ids).map { |a| [a.id, a] }.to_h
+
+      return if account_ids.empty?
+
+      accounts = Account.where(id: account_ids).map { |a| [a.id, a] }.to_h
 
       cached_items.each do |item|
         item.from_account = accounts[item.from_account_id]