Support more language codes for various places

Updated code so that at the lower levels we accept all language codes
supported by unilangs, not just the ones in the unisubs mapping.  For
example now we can have a SubtitleLanguage with language_code="en-us".

We still don't present these as choices to the user, but allowing them
in the model code lets us handle partners that use these codes.

Switched code from using settings.ALL_LANGUAGES, to the constants in
utils.translations.  The utils.translations variables are easier to
use and also differentiate between supported languages and all
languages.

Clear the team readable/writable langs cache since this will
invalidate the old values.

Removed some cruft from the tern days.
Fixed conflict in optional repo commit id
Merge branch 'language-code-choices' into staging