Handle RPM name changes gracefully

Right now: if the package name changes (e.g. 'gitlab' -> 'gitlab-ce', or 'gitlab-ce' -> 'gitlab-ee'), then the RPM post-remove script of the old packages runs last, and removes symlinks from /usr/bin (e.g. /usr/bin/gitlab-ctl).

This sucks.

We added a message for the user to help them deal with this in !331 (merged) but we might want to look for something more robust. (Make the symlink part of the package, instead of creating it with a script?)

cc @marin @sytses