Backblaze do not offer versioned downloads of their installer so
attempting to upgrade the Cask causes issues with mismatched checksums
if the previous installer is still cached.

This means we lose the ability to detect updates but forcibly upgrading
the Cask (e.g. with `brew cask upgrade backblaze`) should now work as
expected.

Rather than having to manually delete the various applications,
preference panes and `launchctl` jobs installed by the Backblaze
Installer ourselves, use the included Backblaze Uninstaller script
instead.

Note that the Uninstaller will not delete user-specific files so we
retain the existing zap stanza.