Ubuntu Flavors, check onboard gsettings overrides

[Jeremy Bicha]

I synced onboard from Debian today but it's stuck in artful-proposed
for unrelated reasons.

Ubuntu has been customizing onboard's default settings for years. But
now that the package is in sync with Debian, those settings will need
to be moved to the individual flavor's gsettings overrides.

I have opened https://launchpad.net/bugs/1703116 to track this issue.

This was already done for the Ubuntu Desktop and Ubuntu GNOME flavors
in zesty but the change was a bit late in the release cycle so I
postponed dropping the overrides from the onboard package until now.

Thanks,
Jeremy Bicha