This change adds onboarding_start, onboarding_end and onboarding_step
columns to the user_settings table. the first two are used to determine
if a user should go through onboarding, the last will be used to as the
step a user got to before exiting onboarding without finishing.
Issue: https://github.com/storj/storj/issues/5660
Change-Id: I8070c880d0d2fc22086f24087c962f57c695cc50
