Add '' default to bodyclass in courseware templates

This prevents from having a 'None' CSS class in the <body> tag when no
css_class is defined for the course.
Remove warning about LTI not working in Preview mode
Prepend theme template paths

Allow templates override.
MixedModulestore wraps most getters, update_item, delete_item
with code to translate between addressing schemes based on app
and persistence layer addressing scheme specification.

STUD-1206
Added test for defaulting use_locations
update_item and other refactorings continued
Enable (and test) xml export of pure XBlocks

[LMS-179]
[LMS-209]
[LMS-1345]
Review-driven changes (to-be-squashed)
Make i18n:generate more flexible

Now normal i18n:generate doesn't care about missing files, and
i18n:generate_strict does.  The robot task calls the strict version.

There may well be a better way to make the two tasks, but my Ruby+Rake
skillz are weak, and it's all going away soon.
Switch test_xblock_wrappers to use ddt, and to considate logic
Added an alt tag to the footer logo; added charset meta tag; moved
DOCTYPE to the top of the file.
Fix goto_position in sequence coffeescript.

Replace single-quotes with double-quotes so that #{@ajaxUrl} is
interpolated.
Refactor test_xblock_wrappers to use Factories
Rename URL_PATH to url_path in acceptance code

It's not a constant, and some subclasses will want to make it a
function with @property.
Continue updating Appendix E
skip *~ files in import of extra content (e.g. info and about pages)
skip ~ files in static content import
Add acceptance tests for forum response pagination
fix tests that were breaking on dev envs due to TIME_ZONE='UTC'
Merge pull request #2454 from ionis-education-group/theme-template-paths

Prepend theme template paths