Support force-removing a site after deleting associated files

[tomuram]

If a Balsam site is created, and the associated files are removed, the Site cannot be deleted.

$ balsam site init -n x x
$ balsam site ls
ID.       Name            Path            Active
341     x                     /path/to/x   No
$ rm -rf x
$ balsam site rm x
Usage: balsam site rm [OPTIONS] PATH
Try 'balsam site rm --help' for help.

Error: Invalid value for 'PATH': Directory 'x' does not exist.

Expected behavior: The site should be deleted. Balsam attempts to validate the Site directory, which is perfectly reasonable, to avoid unintentionally removing a site from the server. Suggested fix is to add an explicit delete option such as balsam site rm --id 341, which would require the user to know the site id.

Workaround: Create a new site, copy it to the original site directory, change the site id to point to the original site, then remove

$ balsam site init -n y y
$ cp -r y x
$ echo 341 > x/.balsam-site
$ balsam rm x
Do you really want to destroy x?  [y/N]: y
Deleted site x