Prepare Mastodon for zeitwerk autoloader (#15917)

* Prepare Mastodon for zeitwerk autoloader (Rails 6)

Add inflections and rename/move a few classes.

In particular, app/lib/exceptions.rb and app/lib/sanitize_config.rb
were manually loaded while still in autoload paths.

* Add inflection for Url → URL

db/migrate/20160223165855_add_url_to_accounts.rb

@@ -1,4 +1,4 @@
-class AddUrlToAccounts < ActiveRecord::Migration[4.2]
+class AddURLToAccounts < ActiveRecord::Migration[4.2]
   def change
     add_column :accounts, :url, :string, null: true, default: nil
   end