Prepare for syncthing v0.9.0.

This includes chaning the path from github.com/calmh/syncthing to
github.com/syncthing/syncthing, as well as disabling updates from
the web gui
(https://discourse.syncthing.net/t/v0-9-0-upgrade-from-within-gui/537).
Bumpmed version to 0.4.4.