rtl: fix margins in admin
Fix ID check in admin_page_ordering view to prevent some redundant queries.
Add functions mixinto and extensible_meta to mezzanine.boot.

This decorator allows you to dynamically add a mixin class to any
of mezzanine's core classes that use a metaclass of
extensible_meta(__name__). Mixin classes should be decorated with
e.g. @mixinto('mezzanine.pages.models.Page'), and must be defined
before the target model is imported.