PLAT-221

This changes logic to allow more missed use cases of course_image to
work properly.  The cases are:

. XML courses with the course_image attribute set
. Mongo courses that are imported without a contentstore
. Mongo courses that have course_image set but don't have a content store

It also exports default images_static_course.jpg to
images/static_course.jpg to handle a use case where a course author
uploaded an image to the default location in studio without using the
studio interface for adding course images, they then export the course, and then
import it without a contentstore

LMS-1192

STUD-878

Part of STUD-868

in XML deserializations, we can get get floats for start/stop times, so we need to be able to parse those

add test video with float-based end-time

Fixed all common and LMS tests.

The tests were failing because XMLDescriptor adds in some attributes
to _model_data, such as `xml_attributes`, that aren't necessary. The
solution is to handle all XML parsing in VideoDescriptor. There's
still one test failing in CMS, which has to do with metadata being
saved. I'm still working out how to update it in such a way that it
doesn't fail, but still tests something meaningful.

add ability to rewrite links on the old /courses/<org>/<course>/<run>/jump_to/i4x://&lt;org&gt;/&lt;course> .. intracourseware linking to the new portable /jump_to_id/<id> format

change from_xml class method on video class to make sure we honor the fact that the xml string could actually be a pointer to another file on disk which actually contains the attributes

add a default XML serialization if the 'data' field is empty or None. Some XModules now - like video XModule - do not have XML set in the 'data' field

Also fixed things in the toy course where needed

Factories were used when possible but tests depending on importing or things not manageable by factories used the toy course.  New files were added to include the needed functionality.  Most files are blank or have minimal xml needed to work

A blank asset file was also added to toy

This fixes the following tests(T stands for toy, F stands for factory made):
T import_textbook_as_content_element (needed to add a textbook to toy)
F static_tab_reordering
T import polls (needed to add a poll to toy)
T xlint_fails
F delete
T about_overrides (needed to add an about override to toy)
F remove_hide_progress_tab

-- Changed docstrings to use double quote.
-- More consistent naming for youtube IDs (normal_speed_id -> youtube_id_1_0).
-- Formatting changes for readability.
-- Updated test XML to expect JSON-encoded strings; prevents tests from spuriously failing.

Update XML in old video tests. This change tags like <video youtube="1.0:abcdefg"> to <video youtube_id_1_0="abcdefg".

Update XML in old video tests. This change tags like <video youtube="1.0:abcdefg"> to <video youtube_id_1_0="abcdefg".