Run migrations on PR environments

[bencalegari]

Update bin/update-pr-environment to run migrations on PR environments. Prior to this, the PR env workflow was just building the image and deploying it in Terraform without it.

Changes

• run-command now takes an optional workspace flag so we can execute migrations and anything else we'd like on different environments.

Acceptance testing

• Acceptance testing after merge
  • This change is hard to test locally, so we'll test it in the demo environment (deployed automatically after merge.)
  • Make sure to notify the team once this PR is merged so we don't inadvertently deploy the unaccepted change to production. (e.g. :alert: Deploy block! @ffs-eng I just merged PR [#123] and will be doing acceptance testing in demo - please don't deploy until I'm finished!)

[tdooner]

We should make it create a separate copy of the database for the review app, too, or else it's going to break Dev when we delete a column (or something like that) in a review app.