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/20160223165723_add_url_to_statuses.rb

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

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

db/migrate/20160322193748_add_avatar_remote_url_to_accounts.rb

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

db/migrate/20170318214217_add_header_remote_url_to_accounts.rb

@@ -1,4 +1,4 @@
-class AddHeaderRemoteUrlToAccounts < ActiveRecord::Migration[5.0]
+class AddHeaderRemoteURLToAccounts < ActiveRecord::Migration[5.0]
   def change
     add_column :accounts, :header_remote_url, :string, null: false, default: ''
   end

db/migrate/20171130000000_add_embed_url_to_preview_cards.rb

@@ -1,6 +1,6 @@
 require Rails.root.join('lib', 'mastodon', 'migration_helpers')
 
-class AddEmbedUrlToPreviewCards < ActiveRecord::Migration[5.1]
+class AddEmbedURLToPreviewCards < ActiveRecord::Migration[5.1]
   include Mastodon::MigrationHelpers
 
   disable_ddl_transaction!

db/migrate/20180304013859_add_featured_collection_url_to_accounts.rb

@@ -1,4 +1,4 @@
-class AddFeaturedCollectionUrlToAccounts < ActiveRecord::Migration[5.1]
+class AddFeaturedCollectionURLToAccounts < ActiveRecord::Migration[5.1]
   def change
     add_column :accounts, :featured_collection_url, :string
   end

db/migrate/20200529214050_add_devices_url_to_accounts.rb

@@ -1,4 +1,4 @@
-class AddDevicesUrlToAccounts < ActiveRecord::Migration[5.2]
+class AddDevicesURLToAccounts < ActiveRecord::Migration[5.2]
   def change
     add_column :accounts, :devices_url, :string
   end