Branding - customizable logos and skins

This feature branch adds the ability for quickly swapping out logos and creating skins for spreed-webrtc.  Extensive variables where created to give the user more fine grained control in `global/_variables.scss`. Under `global/_branding.scss` a user can change the logo and loading logo. Customizable skins can be put into `global/skins` and imported to be built in `global/_variables.scss`. Two example skins are included.