Update PostgreSQL user permission in Usage Guide

[mcserep]

CodeCompass does not require SUPERUSER privileges in PostgreSQL, only CREATEDB.

Also updating documentation links to more up to date version (9.5 -> 12).

[intjftw]

LGTM!
Related question: should we also update the Postgres version in the CI and the tarball as well? It's kind of inconsistent. For Ubuntu 18.04 we install v. 10 (I know we'll stop supporting it soon but still), and also we build v.10.x in the tarball.

[mcserep]

LGTM! Related question: should we also update the Postgres version in the CI and the tarball as well? It's kind of inconsistent. For Ubuntu 18.04 we install v. 10 (I know we'll stop supporting it soon but still), and also we build v.10.x in the tarball.

I don't think we should spend any effort on updating the Ubuntu 18.04 scripts, as we will drop support for it soon anyways (see #580). Also, in the default package repository of Ubuntu 18.04, PostgreSQL version 10 is available, 12 is not.