Trusted Applications: Disallow invalid Origin values / strip trailing slashes

[angelo-v]

It is a common and frustrating mistake to put a slash at the end of the Origin value (see e.g. https://gitter.im/solid/chat?at=5f201ed23eef7e1ffe3c40b4)

The UI should validate if the value is a valid Origin value, aka Origin: <scheme> "://" <hostname> [ ":" <port> ] (https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Origin)

Especially it should disallow or auto-remove trailing slashes