Large SVG sharing icons, adjust CSS, and simplify admin

Introducing: Large sharing buttons!
 - Scalable vector icons (SVG icons)
 - Mobile ready
 - Retina and high-DPI ready
 - Support for older browsers (reverts gracefully to PNG)
 - Customizable through CSS code (effects, height & width, border-radius, and much more)
 - Large icons are available for the most popular services right now. More services will roll out soon
Simplify, reorganize admin options
Add Icon Size option to switch between large SVG icons and traditional PNG icons
Rename Additional HTML to Service Buttons HTML
Adjust CSS for large icons