Enhancement: Make snapshot directory structure of `mount` command customizable

We've added the possibility to customize the snapshot directory structure of
the `mount` command using templates passed to the `--snapshot-template` option.
The formatting of snapshots' timestamps is now controlled using `--time-template`
and supports subdirectories to for example group snapshots by year. Please
see `restic help mount` for further details.

Characters in tag names which are not allowed in a filename are replaced by
underscores `_`. For example a tag `foo/bar` will result in a directory name
of `foo_bar`.

https://github.com/restic/restic/issues/2907
https://github.com/restic/restic/pull/2913
https://github.com/restic/restic/pull/3691
