Add Install section to gitlab-backup.timer
Prettify gitlab.target

Add a install section, some comments that used to be in there before
and use a variable for the systemd directory in the PKGBUILD.
Do not require a gitlab user to build.

Rely on the gitlab(-shell).install to fix the permissions. We did
that anyway since the gitlab user on the build machine is bound to
have a different UID/GID from the one on the install machine.