Add a setting for the site favicon

[revi]

Mostly low-priority cosmetic request: when I have a LOT of tabs (like I do here), I would like to differentiate the tabs based on the icon. (and cosmetic reasons too)

[samwilson]

This is a great idea. I wonder what the best way to do it is.

One simple way to do this is for site admins just to add a favicon.ico to the public/ directory. It doesn't have to be managed by the app at all.

https://favicon.io/ is an easy way to create the file.

There are other sizes and types of favicon that can be useful, so it's still worth adding support to Twyne, I think.

[revi]

Maybe bundle a default favicon somewhere (generated via favicon.io or something) and presence of public/favicon.ico overriding the default favicon? Or just display favicon when public/favicon.ico without the bundled favicon? Also while we are at public directory... I got some idea for... #178 .