DB Deployment Scripts and Variables

[MinhxNguyen7]

Description

• Add scripts to add migration step to deployment.
• Add DB connection strings for staging and prod to GitHub Actions, corresponding to Actions Secrets.
• Additionally, we had to create zotmeet-prod and zotmeet-staging users with relevant permissions to do the migrations. More info coming in Document DB Deployment/Migrations Troubleshooting #87

Testing

• See that the staging deployment on this PR works.
  • The only existing functionality that uses the DB is signing in. We can see that we get a Postgres error that indicates that at least we are capable of accessing and inserting into the DB.
• See that the prod deployment works after merging.

Issues

• Closes Dev/Prod DBs and environment variables for deployment #39

Future Follow-Up

• Creating a new user doesn't work right now, per Can't sign up #88.

[seancfong]

LGTM