Textbooks accessibility changes

Removed image-based textbook info from XML tutorial.

Added not about accessibility and textbooks to Building a Course file.
Updated change log
Modify jquery.timeago to better support i18n

Use string interpolation instead of concatenation of prefix, content,
and suffix. This is motivated by the fact that Transifex does not accept
empty strings for translation (even when pgettext is used to provide
context).
visual styling changes to components and static pages in order to use updated icon button styling for duplicate and delete actions
adjusted unit_spec.js to reflect new HTML structure, and corrected screenreader label for delete button
Correct three typos in video documentation
updated static pages tests to reflect html element change in component.html
Update tolerance information for Numerical Input problems
Re-add .mpeg as possible video type
Allow user with BetaTester role correctly use LTI. BLD-641.
Fixed flaky test for instructor command openended_post.

The submission time is serialized into json and sent to xqueue. Previously the
whole json body argument to send_to_queue() was being matched. Sometimes the
time calculated in the test would differ slightly from the time calculated in
send_to_grader() and the test would fail. This fix tests individual fields of
the body argument to mock_send_to_queue() instead.
Merge pull request #2211 from edx/oleg/beta_user_for_lti

Allow user with BetaTester role correctly use LTI. BLD-641.
Merge pull request #2282 from edx/sylvia/documentation/DOC-12

Correct three typos in video documentation
Merge pull request #2289 from edx/sylvia/documentation/BLD-671

Update tolerance information for Numerical Input problems
Merge pull request #2291 from edx/usman/fix-openended-post-command-tests

Fixed flaky test for instructor command openended_post.
Merge pull request #2293 from edx/markhoeber/documentation/stud-1130

Markhoeber/documentation/stud 1130
Merge pull request #2267 from edx/gprice/timeago-better-i18n

Modify jquery.timeago to better support i18n
fixed typo in comment for assets.scss
Merge pull request #2276 from edx/studio/duplicate/visuals

Updates Studio's visual styling of action buttons on components and static pages
add middleware to be able to expire inactive sessions after N seconds

configure middleware

add test for session inactive timeouts

add Studio inactive session timeout test

change login method used

add create_test_account to test

make sure the expected redirect URL is right

fix indenting problem

fix doc string since we moved from minutes to seconds

use utility methods rather than calling another test to set up and activate an account

clean up code violations

respond to PR feedback

use optional params to make code cleaner

pylint fix on test files