These two tests are used when checking if a new page type can be created
under a page instance, or if an existing page type can move under a page
instance.
`is_creatable` better reflects what it is used for, and stops any
confusion between Wagtail's `is_abstract` and Django's `Meta.abstract`.
`is_creatable` takes in to account `Meta.abstract` now, so developers
will no longer need to set both `is_abstract` and `Meta.abstract`.
Documentation for the new attribute has been added, as well as tests.