Update touch icon

Specs on touch icons have changed with higher-res Android devices and the newer iPhones.

- Updated touch-icon sizes to support newer devices
- Removed baked in rounded corners on icon (let the OS add the corners)
- Added non-Apple specific link tag for newer Android/Chrome use

References
- http://www.kylejlarson.com/blog/2013/adding-an-icon-for-iphone-ipad-android-to-your-website/
- https://mathiasbynens.be/notes/touch-icons