Fix pylint violations from #2129
Certs - bugfix cert_whitelist output

* output after cert_whitelist -a or -d purported to be the whitelist entries
  for the requested course, but in fact were all of the entries in every
  course. This commit fixes that.
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
BLD-237: Persist speed preferences between videos.
Use subprocess.STDOUT, rather than sys.STDOUT to rebind stderr
i18n xmodule/course_module.py

LMS-1747
1st draft external graders doc
Update tolerance information for Numerical Input problems
Re-add .mpeg as possible video type
Use yaml for a civilized config file.
I18n segmenting tool.

Messages can be segmented into different .po files based on the location
of their occurrences.  This is so Studio strings can go into different
.po files so we can direct Transifex workers to the LMS strings we
really need done.
A few improvements to extract.

Use the configuration more, and we don't need to delete files we're
about to create.
i18n: extract and generate properly handle segments
Ignore fr and ko_KR, so we can play around with them while getting our ducks in a row.