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.
Eliminate unused import and gratuitous prints
i18n Sequence XModule

LMS-1758
Always add Plural metadata to dummy translations.

When this was conditional, we'd have plural info in some files, not in
others. Then we'd merge them, and msgcat would add wacky markers to
indicate the difference, and the metadata wouldn't parse.
Tighten up the extraction I/O

We were writing the main file twice.  No need.
Collect coverage for bok-choy tests
Merge pull request #2290 from edx/jrbl/mdx_stop_printing

Eliminate unused import and gratuitous prints
Merge pull request #2227 from edx/jrbl/fix_cert_whitelist_report

Certs - bugfix cert_whitelist output
Merge pull request #2295 from edx/sarina/i19-xmodules

i18n Sequence XModule
Merge pull request #2301 from edx/will/bok-choy-coverage

Collect coverage for bok-choy tests
BLD-699: Fix i18n.
Merge pull request #2243 from edx/anton/video-i18n

Internationalize the video player.
Give numerical response tolerance as a range (for example [5,7)) (BLD-25)
Merge pull request #2231 from edx/alex/numtolerance_range

Alex/numtolerance range
BLD-711: Fix offest in FF.
Merge pull request #2303 from edx/ned/i18n-tool-fixes

Ned/i18n tool fixes
Merge pull request #2315 from edx/anton/fix-imageresponse-offset-in-ff

Imageresponse: fix offset in FF.
Add the ability to dark-launch languages

To mark a language as dark-launched, add it to the DARK_LANGUAGES
django conf setting. To activate a dark-launched language, set he
query parameter `preview-lang` to the language code on any url.

[LMS-2045]
[LMS-2077]
[LMS-2076]
Add config_model, a library for database backed configuration

ConfigurationModels can be managed using the admin site. They are
append-only, and track the user who is making the change, and the time
that the change was made. The configuration is stored in the database,
and cached for performance.

[LMS-1220]
Switch dark_lang to database backed configuration