Generate a materialized view for gameRatingsSandbox0

[dfabulich]

Now we'll use gameRatingsSandbox0_mv for pretty much all purposes.

Fixes #478 and fixes #472

[salty-horse]

Is embargopastdate supposed to be used anywhere? I assume it's in the definition of the view that's not part of the repository.

Perhaps both the view and the cron jobs should be documented somewhere.

[dfabulich]

I added more comments about embargopastdate to refresh-embargoed-reviews.

// This URL will be pinged on a regular schedule via a cron job.

// This cron job is really strange.

// The purpose of the cron job is to keep the `gameRatingsSandbox0_mv`
// materialized view up to date as time passes.

// The `gameRatingsSandbox0` view automatically excludes reviews with an
// embargo date in the future; thus, the view is like a `join` of the
// `reviews` table and the current datetime.

// But the trigger that keeps `gameRatingsSandbox0_mv` updated only runs
// when we `update` the `reviews` table. The trigger doesn't naturally
// update the MV as time passes.

// This cron job incorporates the current datetime into the materialized
// view by translating the passage of time into `update` statements on the
// `reviews` table.

// The `reviews` table has an `embargopastdate` column, which serves no
// real purpose other than to be a column that we can update in order to
// activate the MV update trigger. We only read `embargopastdate` here,
// in this query, just in order to find recently unembargoed reviews that
// we haven't incorporated into the MV.

// It's kind of like running `touch` on a file, updating its lastmodified
// date, in order to convince `make` to do something with it. We're
// "touching" the `embargopastdate` in order to activate the post-update
// trigger, refreshing the MV.