Move defaults from yaml templates to field definitions.

This standardizes the XModule field default values to be the same as the
values that are presented by studio when a component is added to a
course.

TODO: Make from_xml in descriptors that had defaults changed give the
old defaults for xml courses
No longer persist XModule templates

Instead, we use XModule field default values when creating an empty
XModule. Driven by this use case, we also allow for XModules to be
created in memory without being persisted to the database at all. This
necessitates a change to the Modulestore api, replacing clone_item with
create_draft and save_xmodule.