Create readonly postgres user for solrupdater

[cdrini]

Description

Fix: Creates a readonly postgres user for solrupdater so there's a stronger guarantee that when we run this on logs ~1mo out-of-date (for the last phase of #1843 ) it won't be causing any unintentional database edits.

Technical

• In our local environment, the default authentication for users is peer; this requires us to create a system user but means we don't need a password (good for dev). On production we'll have to give the postgres user a password somehow?
• Solution based on https://gist.github.com/oinopion/4a207726edba8b99fd0be31cb28124d0
• Note that it's a readcreate user; it can still create new tables. Removing this seemed like more trouble than it's worth (especially if we want to keep prod in sync), so didn't bother.

Testing

• I tried running web.ctx.site.save, but this never worked from solrupdater because there's no ol account associated with it.
• I used the debugger to ensure that when it connects to infobase it connects with username solrupdater
• I tried giving the new postgres user 0 permissions, and new-solr-updater failed (as expected).

[mekarpeles]

lgtm

[tfmorris]

LGTM (confirming what I communicated via Slack)