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.

molecule/default/converge.yml

@@ -12,7 +12,6 @@
         funkwhale_ssl_cert_path: /certs/test.crt
         funkwhale_ssl_key_path: /certs/test.key
         funkwhale_hostname: yourdomain.funkwhale
-        funkwhale_version: 0.19.0-rc2
         funkwhale_env_vars:
           - EMAIL_CONFIG=smtp+tls://user@:password@youremail.host:587
           - DEFAULT_FROM_EMAIL=noreply@yourdomain.funkwhale