FIX: Update and rebake uses of the old centralized avatar service (#16086)

This URL was originally updated in 89cb537fae. However, some sites are not using the proxy, and have configured their forum to hotlink images directly to avatars.discourse.org.

We intend to shut down this domain in favor of `avatars.discourse-cdn.com`, so this migration will re-write any matching site setting values and queue affected posts for rebaking.

db/migrate/20220302163246_update_avatar_service_domain.rb

@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+class UpdateAvatarServiceDomain < ActiveRecord::Migration[6.1]
+  def up
+    existing_value = DB.query_single("SELECT value FROM site_settings WHERE name = 'external_system_avatars_url'")&.[](0)
+
+    if existing_value&.include?("avatars.discourse.org")
+      new_value = DB.query_single(<<~SQL)&.[](0)
+        UPDATE site_settings
+        SET value = REPLACE(value, 'avatars.discourse.org', 'avatars.discourse-cdn.com')
+        WHERE name = 'external_system_avatars_url'
+        AND value LIKE '%avatars.discourse.org%'
+        RETURNING value
+      SQL
+
+      DB.exec <<~SQL, previous: existing_value, new: new_value
+        INSERT INTO user_histories
+        (action, subject, previous_value, new_value, admin_only, updated_at, created_at, acting_user_id)
+        VALUES (3, 'external_system_avatars_url', :previous, :new, true, NOW(), NOW(), -1)
+      SQL
+    end
+  end
+
+  def down
+    # Nothing to do
+  end
+end