Add fallback to logged in user properties in route config

[jdalsem]

We have different implementations at the moment where $username or $guid is part of the url path in the route configuration.

Sometimes it is optional and we then cannot add gatekeeper middleware because we implement default behaviour in the resource -> we mostly default to logged in user and then check if it is a user.

It would make things easier if the route username/guid is required, but can default to the logged in user. That way we can use middleware and have consistency in default logic.

One additional pattern (for example with /settings/user/foo_non_existing) is where a username is required, but is not required to be a valid user. This pattern needs to disappear. Passing an invalid username should result in a 404 (not fallback to another logged in user).