Support funkwhale_version=latest to use current release

Changes funkwhale_version to support and default to latest, which installs the version set by https://docs.funkwhale.audio/latest.txt

This assumes there will never be a git branch named latest. I've tested (unset), "1.1.4", and "latest" with zip distributions. I haven't tested build from source.

CI failure is pre-existing on master.