Support xblocks with children on the container page.

STUD-1363
Use display_name_with_default instead of display_name.
Increase max-width for breadcrumb trail.
Improve code clarity.
Merge pull request #2807 from edx/christina/containers

Support xblocks with children on the container page.
Fixes tinymce bug:
    On using the image plugin, the image was not appearing in the
    visual editor immediately after using the command.

    Selenium test added.
making mixed ms capable of front ending all modulestores for most operations including CRUD
STUD-600
Mixed no longer translates
unicode correctly and understandably
Pass user id to update_item
Point to prefer_xmodules new locn
Better sorting of stores for get_courses
May apply to all store ordering
Mixed modulestore handles most modulestore functions, cont.
Change get_courses default branch to draft.
We need to figure out how apps configure what default they want or ensure they get the one they want
No longer create loc maps on create_(item|course)
pylint and pep8 fixes
Test exception on read-only db
Requesting an xmodule field in wrong app context raises exception rather than assertion
Add tests for get_orphans
Test special code paths in create_item