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.

Rake by default imports all .rake files in the rakelib dir, so we can
use that rather than doing our own import loop.

Using `task()` reopens the definition of the task, and all we really
need to do is get a reference to the task itself to invoke it.

The LMS/CMS server startup depends on the :assets task, which needs
to receive :system and :env arguments. Since the existing server
startup tasks didn't take a :system argument, a new command called
:runserver is created that does take :system, :env, and :options
arguments. The old server startup tasks are adjusted to wrap the one
:runserver task that does all of the heavy lifting.

Make the externally facing asset watch commands wait for user intervention, and add internal tasks that only run as long as the rake process