CON 1888 - Media support on Venus

Support for 

* Galleries
* Slideshow
* Slider
* New galleries
* Videos

File: mediawiki.basic.legacy.css is part of /skins/common/shared.css
Did you consider combining this selector with https://github.com/Wikia/app/pull/5041/files#diff-1be6d2966ddf79eb28870a31f2cfe545R61