Better favicon handling

[pixelastic]

• [x] Each theme should have its own favicon
• [x] A favicon.ico should always be created
• [x] favicon.png should be used in precedence to favicon.svg if present
• [x] The documentation should have a template to copy to create its own favicon.svg file

[pixelastic]

• Each theme has its own favicon.svg and favicon.ico
• The .ico is never referenced, but browsers still request it
• The .svg is a wrapper around an emoji
• If people want to use another extension, they need to update _data/meta.json
• Documentation has been updated

This might "break" some sites that will now use default favicons until their _data/meta.json is updated