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
adding db overrides to aws_migrate.py
remove check for PASS
adding check back in
check for None so an empty string is valid for a password
adding DB_MIGRATION_ENGINE and fixing defaults
fixing syntax
Prepend theme template paths

Allow templates override.
Make assets:coffee a dependency of i18n:extract

Extraction is done on the compiled .js files, so coffee compilation
should be done before extracting strings.
Enable (and test) xml export of pure XBlocks

[LMS-179]
[LMS-209]
[LMS-1345]
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
Start datetime localization in LMS

Currently just coerces the time displayed for due dates
(accordion.html and progess.html) to settings.TIME_ZONE
fixing create-dev-env.sh to use Brewfile
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.
Merge branch 'master' of https://github.com/edx/edx-platform
Refactor test_xblock_wrappers to use Factories
Clarify DATA_DIR path