Check if web root has already been appended

This whole `linkTo`, `linkToAbsolute`, `linkToRemote`, `linkToRemoteBase` etc. etc. is a mess. We should move it all to the UrlGenerator class, decide on a standardized way and obsolete all the "helper" functions.

This is just a hack to make it work.

@PVince81 @jancborchardt @MorrisJobke