add option to start a slot from a user selected commit of LHCbNightlyConf
The page https://lhcb-nightlies.web.cern.ch/nightly/start_slots uses, by default, the latest commit on the default branch of LHCBNightlyConf (regularly updated), but sometimes it is useful to use the version from a different branch or pick up the latest commit before the web page gets updated.