installer: Add support for custom database user and dbname.
installer: Add support for custom database user and dbname.

Add support for custom database names and database users, which can be
set with the `--postgresql-database-name` and
`--postgresql-database-user` install script options. If these
parameters aren't provided, then the defaults remain "zulip".

Fixes #17662.

Co-authored-by: Alex Vandiver <889941edc7929c0c16e8b429f61a0accebf17aa6@zulip.com>

kou/zulip,hackerkid/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,punchagan/zulip,punchagan/zulip,andersk/zulip,hackerkid/zulip,eeshangarg/zulip,rht/zulip,eeshangarg/zulip,rht/zulip,zulip/zulip,hackerkid/zulip,andersk/zulip,eeshangarg/zulip,rht/zulip,eeshangarg/zulip,zulip/zulip,eeshangarg/zulip,punchagan/zulip,hackerkid/zulip,andersk/zulip,rht/zulip,kou/zulip,kou/zulip,punchagan/zulip,zulip/zulip,rht/zulip,andersk/zulip,andersk/zulip,punchagan/zulip,rht/zulip,punchagan/zulip,rht/zulip,kou/zulip,andersk/zulip,kou/zulip,zulip/zulip,zulip/zulip,andersk/zulip,kou/zulip,zulip/zulip,hackerkid/zulip,hackerkid/zulip,eeshangarg/zulip,hackerkid/zulip,punchagan/zulip