Bugfix: Allow usernames containing underscore and more

The security fix in rest-server 0.11.0 (#131) disallowed usernames containing
and underscore "_". The list of allowed characters has now been changed to
include Unicode characters, numbers, "_", "-", "." and "@".

https://github.com/restic/restic/issues/183
https://github.com/restic/restic/pull/184