Switch from accepts to fluent-langneg for language header parsing

[jaredhirsch]

We currently use the 'accepts' npm library to get an ordered list of user-preferred languages from a request.

The 'fluent-langneg' library just added an API in 0.0.3 to parse an accept-language header, a function called acceptedLanguages.

We should switch to dogfooding the new 'fluent-langneg' API.

I'm not sure the output will be the same in all cases, particularly the case of "Accept-Language: *", so we'll need to test that. Manual testing is fine; headers can be tweaked from FF devtools. Unit tests would be swell. Also note that an express request object is passed into accepts, while just the header should be passed into acceptedLanguages.

[ianb]

Looks like we're doing this now