Enhancement: Implement web server to browse snapshots

Currently the canonical way of browsing a repository's snapshots to view
or restore files is `mount`. Unfortunately `mount` depends on fuse which
is not available on all operating systems.

The new `restic serve` command presents a web interface to browse a
repository's snapshots. It allows to view and download files individually
or as a group (as a tar archive) from snapshots.

https://github.com/restic/restic/pull/4276
https://github.com/restic/restic/issues/60