Automatically run migrations before automatically deploying

[pcraig3]

Summary

We want to automatically run migrations when we deploy the app.

This PR creates a new migrations action that automatically runs all migrations in the app.

It also has a new main.yml file which manages when the jobs actually are run.

Previously:

• ci: run test quite. Would run on all PRs and on merges to main.
• deploy: deploy new container. Would run on all merges to main.

Now:

• ci: run test quite. Would run on all PRs and on merges to main.
• migrate: migrate DB. Runs on merges to main, only if ci has succeeded.
• deploy: deploy new container. Runs on merges to main only if migrate has succeeded.