MAIN-3096: Pass correct path-prefix when making a request to Vignette

Vignette requires to pass correct path-prefix to it when sending a request. For unknown new galleries in VE were not tested in different languages than EN and this problem didn't pop out (path-prefix for English is null).

This whole approach is sort of temporary and will be "re-done" when we get a proper Vignette library on the client side.

https://wikia-inc.atlassian.net/browse/MAIN-3096