Enhancement: Support group accessible repositories

Rest-server now supports making repositories accessible to the filesystem group
by setting the `--group-accessible-repos` option. Note that permissions of
existing files are not modified. To allow the group to read and write file,
use a umask of `007`. To only grant read access use `027`. To make an existing
repository group-accessible, use `chmod -R g+rwX /path/to/repo`.

https://github.com/restic/rest-server/issues/189
https://github.com/restic/rest-server/pull/308