Remove warning about LTI not working in Preview mode
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
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'
Add test for ignoring tilde files on import of extra content
Add test for ignoring tilde static files on import
Reorganize content in Appendix E

Change order of tags (now alphabetical); add link list at beginning of document
Merge pull request #2513 from edx/sylvia/documentation/BLD-801

Sylvia/documentation/bld 801
Merge pull request #2485 from edx/ned/make-i18n-generate-forgiving

Make i18n:generate more flexible
Merge pull request #2465 from glowe/fix-sequence-position

Fix goto_position in sequence coffeescript.
Merge pull request #2504 from edx/gprice/forum-pagination-acceptance

Add acceptance tests for forum response pagination
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.