Fix PostgreSQL configuration for systemd

From #1143 (moved):

Currently for legacy reasons, the postgres user on GitLab.com is using UID/GID of 1101/1101, which is a non system user account.

This is a really critical issue for anyone using PostgreSQL 9.6: https://wiki.postgresql.org/wiki/Systemd. The documentation suggests one of two ways to fix this:

1. Make the gitlab-psql user a system user (i.e. UID from 100-500)
2. Add RemoveIPC=no to /etc/systemd/logind.conf

Failure to do this will cause PostgreSQL to crash if the gitlab-psql user is logged out.

I think this is really important to address, blog, and document for everyone using 9.6.