Complete upgrade to latest stable/postgresql chart

This is mostly implemented in !608 (closed)

Still to do:

• Use the current latest version of the chart (6.3.0 at the time this issue was opened)
• With postgresqlUsername set to gitlab, the pg_trgm extension fails to be loaded into the database since the user is not a superuser. We should use the default postgres username for postgresqlUsername, and add the gitlab user later as a script in the initdbConfigmap. The goal is to be able to keep the gitlab user as a non-superuser account. This will also require splitting out the password for the gitlab user to a different value than postgres user.
• image.tag value should default to match what is in omnibus-gitlab (10.9 at the time this issue was created).