Support external PostgreSQL

Adds support for external PostgreSQL by only deploying PostgreSQL resources if postgresql.install=true.

Closes #183 (closed)

Test instructions

1. Deploy Bitnami's PostgreSQL Helm Chart to the cluster

2. Create the role & DB, and enable the required extensions

3. Create secret for PG password:

   $ kubectl -n gitlab-system create secret generic externalpg-password --from-literal=password=abcd1234

4. Deploy the GitLab CR:

    apiVersion: apps.gitlab.com/v1beta1
    kind: GitLab
    metadata:
      name: gitlab
    spec:
      chart:
        version: "4.11.3"
        values:
          certmanager-issuer:
            email: you@email.com
          global:
            hosts:
              domain: yourdomain.com
            ingress:
              configureCertmanager: true
            psql:
              host: externalpg-postgresql.gitlab-system.svc.cluster.local
              password:
                secret: externalpg-password
                key: password
          postgresql:
            install: false

5. Ensure that migrations pass and the instance comes up and is usable.