Addition of 3 TinyMCE plugins

The following plugins added to toolbar in Edit of Unit:
 - fontselect
 - forecolor
 - image
Remove unused massemail commands.
Remove warning about LTI not working in Preview mode
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)
Switch test_xblock_wrappers to use ddt, and to considate logic
fixed conflict between Annotator and internationalitation
Refactor test_xblock_wrappers to use Factories
skip *~ files in import of extra content (e.g. info and about pages)
skip ~ files in static content import
fix tests that were breaking on dev envs due to TIME_ZONE='UTC'
Add pycharm helpers to gitignore
Add test for ignoring tilde files on import of extra content
Add test for ignoring tilde static files on import
Avoid requesting unnecessary thread responses

The front-end code for rendering a thread always makes an AJAX request
for the resopnses, regardless of how the thread was accessed (directly
by URL or by selecting it from the thread list). Previously, when
a user accessed a thread directly by URL, the LMS would initially
request the thread from the comments service with responses, but that
response data was never actually used. Now, the responses are not
fetched with that initial query.
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.
Image Modal CMS HTML Block subtype
- Added YAML file for the HTML template code for the modal to work
- Added CSS and JS code for modal to look and function properly
- Updated code to take comments into account.
- Simplified HTML template and expanded JS to set up image modal on load.
- Added preliminary drag script.
- Converted jQuery UI draggable to Draggabilly