doc: rewrite PostgreSQL authorization instructions
    
    Rewrite the PostgreSQL authorization instructions in the Server HowTo
    guide.
    
    Provide two complete examples: the "all-in-one localhost" option, and
    the "separate postgres over the network" option. This reduces the
    decision trees and makes it easier to copy and paste the entire
    pg_hba.conf file. Explain what each mystery setting does and link to the
    PostgreSQL documentation.
    
    Only set listen_addresses to "*" if we need it, and turn it off if we
    don't.
    
    Simplify the instructions so that the user only restarts the postgresql
    daemon once.
    
    Clarify that koji-web does not need DB access, and the hub is the only
    service that requires direct DB access.
    
    Remove the example of authorizing the apache system user, because that
    implies that the admin must run a series of complicated GRANT
    instructions to allow both koji and apache to write to the DB. Just
    grant the koji user full trust access for simplicity.