VE-1704: Button styling for Venus

Previously, Oasis buttons were extended for VisualEditor. Now, we need to handle extending of buttons as a skin-specific option. 

ve.init.mw.WikiaViewPageTarget.init.js now loads Venus or Oasis-specific stylesheets. Both of those stylesheets import VisualEditor.scss as well as specific button styling. 
Agree that comment should be fixed. We forgot about it introducing changes here: https://github.com/Wikia/app/commit/c5a110b40e802b7521ef39ccf80ca7543a0c9657

Can we try to find different className for this container? Current suggest that element we match is button.