Debian postinst: check for present user/group before adding them

In git commit 7216aab0d the check for present user was removed
and therefore upgrading the Debian package on existing Jenkins
installations fails. Also the newly added code for adding a
configurable Jenkins group is missing a check for a possibly
already present Jenkins group.

Closes: JENKINS-22710
[FIXED JENKINS-22715] Merge pull request #1208 from mika/mika/fix_debian_upgrades

Debian postinst: check for present user/group before adding them
Show full display name of SCMedItem on polling status page.
[FIXED JENKINS-22686] Add a few pixels distance between tick labels.
[JENKINS-22686] Don't use fixed pixel count for label distance.

Instead, use a distance of 0.5x the width/height of the label.

This improves appearance in combination with JENKINS-22688 for
different font sizes.
Noting #1208 and #1199