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.

index.html

@@ -11,7 +11,7 @@ permalink: index.html
                         <div class="monero-video">
                             <!--iframe width="560" height="315" src="https://www.youtube.com/embed/TZi9xx6aiuY" frameborder="0" allowfullscreen></iframe-->
                             <video controls poster="/img/monero-community.png" onclick="this.paused ? this.play() : this.pause();" preload="none">
-                                <source src="/media/Monero_Promo.m4v">
+                                <source src={% if site.lang == 'en' %}"/media/Monero_Promo.m4v"{% else %}"/media/{{ site.lang }}/Monero_Promo.m4v"{% endif %}>
                             </video>
                         </div>