[Enhancement] Use proper favicons

[daviddarnes]

Previously, the only form of site icons / logos was the logo and the avatarurl keys in the config. The avatarurl was taking advantage of the image resizing provided by Gravatar. Collectively this all came with multiple issues:

• You can't use an SVG image in the app manifest, you need raster images
• You may not be using Gravatar to represent your website
• You couldn't control what favicons you have on the site
• You couldn't bundle the icons into the project easily

These proposed changes allow for More fine tuned control of favicons and manifest icon sizes. It also fixes the PWA error when using an SVG for a manifest icon. For legacy users avatarurl will still be available but only for the shortcut icon meta item.