[v0.9.0] Upgrade from within GUI

v0.9.0 includes the ability to perform upgrades directory from the GUI. The check is performed when the GUI is loaded and if a newer version is available an upgrade button is shown in the top bar.

For those of you building syncthing for environments where this is not desirable (Android, package managers, etc): ./build.sh "" -tags noupgrade or make sure that the Go compiler gets -tags noupgrade in some other way. This removes both the upgrade check in the GUI and the syncthing -upgrade option.