Configurable home-directory for sftp-users

Right now sftp `copyparty:` and `copyparty:/` both point to the same path. But it'd be kind of neat to be able to configure where the one without a `/` goes.

maybe just

`sftp-key: foo:/u/foo `

or something like that? Or maybe not, since one might want to use that kind of syntax for potential feature of restricting access paths for a key, or something like that

`sftp-key: foo?h=/u/foo ` maybe?` or a global option like

`sftp-home: /u/${u}`

though I suspect ssh keys might be given to services without a logical home-directory..

Not even close to a necessary feature of course, but a small usability improvement..

Takk igjen!