Add Link to Transifex

[jjeffryes]

To increase the number of contributions of translated text, we should add a link to Transifex along with explanatory text next to the language selection controls in Settings.

It's likely most users don't think about translations until selecting a language, if they notice phrases in their language are not translated it's not obvious how they can help to fix that.

https://www.transifex.com/ob1/openbazaar/