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
Review-driven changes (to-be-squashed)
Add pycharm helpers to gitignore
For a11y, modal dialogs and script templates should be hidden from
screen readers. Also, the mathjax message should be wrapped to prevent
non-IE browsers from complaining about a feature that is IE-only.
More than one dummy language
Latest eo strings, lots of small differences due to Dummy changes
Updated title and template for image modal html component

LMS-2215
Merge pull request #2522 from edx/markhoeber/image-modal-title-change

Updated title and template for image modal html component
Fix pylint violations
Merge pull request #2511 from edx/zoldak/pycharm

Add pycharm helpers to gitignore
Remove unused imports.
Merge pull request #2540 from cpennington/fix-pylint-errors

Fix pylint violations
Improved sequence navigation accessibility
Add Greek language translations
Merge pull request #2496 from edx/dcs/a11y-hide-things

For a11y, modal dialogs and script templates should be hidden...
Refactor student views

Fix pylint/pep8 warnings, use JsonResponse instead of HttpResponse where useful,
put in TODOs to change HTTP status codes to be more accurate.
Merge pull request #2498 from edx/dcs/a11y-sequence-nav

Accessibility improvements to the sequence navigation
Merge pull request #2541 from edx/ned/remove-unused-imports

Remove unused imports.