Add possibility to play localized videos + welcome video in Russian

This commit add the possibility to add videos with the voice-over in different languages. The videos should be uploaded in /media/LANG/, where LANG is the language code, and should have the same name and extension of the original in English.

This commit also adds the "Monero Promo" video with voice-over in Russian, made by the xmr.ru community.

resolves #952 (closed)