Sync with Transifex and Crowdin

[ghost]

Transifex and Crowdin are widely used by other OSS projects in order to accommodate projects wanting to migrate to Pootle it would be nice if Pootle could sync in data from those platforms while still allowing contributors from those platforms to use those platforms.

[dwaynebailey]

Hey @bkerensa, this is something we're really interested in. We can certainly help with individual projects as a step towards making this possible for other users. Feel free to chat to us on our channel https://gitter.im/translate/pootle