Add release phase to Heroku deployments

[mikej]

Description of the issue 📄

Want to be able to have migrations run automatically before new deployments
Based on the instructions here: https://devcenter.heroku.com/articles/release-phase

To do 📋

• Claim this issue (comment below, or assign yourself if you are part of the codebar org)
• Fork and clone the repository
• Update the relevant files. Follow the steps to fix section in this issue.
• Commit your changes as one commit. Use the title of this issue as your commit message
• Submit a pull request
• Mention this issue in the PR description by including it's number
• Have your pull request reviewed & merged by a codebar team member