Enable (and test) xml export of pure XBlocks

[LMS-179]
[LMS-209]
[LMS-1345]
Switch test_xblock_wrappers to use ddt, and to considate logic
Refactor test_xblock_wrappers to use Factories
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
Merge pull request #2362 from edx/giulio/image-modal-squash

Image Modal CMS HTML Block subtype
Missed some places to change the registration response status code

LMS-2203
Merge pull request #2427 from edx/sylvia/documentation/BLD-793

Remove warning about LTI not working in Preview mode
Add the ability to view templates in a browser in development mode

This adds a simple mechanism to view any template file directly in a browser. The intention is that we can use this to build up reference HTML files which illustrate common UX patterns. We can then view the resulting pages without having to have a working implementation.

In development mode, the URL is /template followed by the path to the template file. For example, you can view the 404 page at:

    /template/404.html

You can also supply string parameters to the template by specifying them as query parameters in the URL.

e.g.  /template/my_template?name=Foo
Merge pull request #2514 from edx/gprice/fix-single-thread-recursive

Avoid requesting unnecessary thread responses
Add README to tilde fixture, fix pylint/pep8 issues
Merge pull request #2510 from edx/jbau/fix-timezone-test-breakage

fix tests that were breaking on dev envs due to TIME_ZONE='UTC'
Merge pull request #2413 from edx/bugfix/ichuang/do-not-import-tilde-files

Skip tilde files when importing
Merge pull request #2524 from edx/andya/dev-show-template

Add the ability to view templates in a browser in development mode
Add tests of XML exporting to test_xblock_wrappers
i18n word cloud LMS-2197