Eliminate unused import and gratuitous prints
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.
i18n Sequence XModule

LMS-1758
Collect coverage for bok-choy tests
BLD-699: Fix i18n.
Give numerical response tolerance as a range (for example [5,7)) (BLD-25)
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.
BLD-711: Fix offest 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