Spec: https://html.spec.whatwg.org/multipage/system-state.html#custom-handlers

These API's allow websites to register a callback for specific schemes. Implementing the web API is relatively straightforward. The interesting part is hooking this up to embedders so that they can specify handlers however they want.

Additionally, we also need to persist the information so that we don't constantly nag the user about it. We can leave that up to the embedder, but it would be nicer if Servo handles that.