run database migrations on hot-upgrade deploys

[vanderhoop]

As a maintainer,
I want to automatically run database migrations when pushing hot upgrades,
So that we don't have to run full-restart deploys when migrations are added.

By Way Of Explanation: at the moment, we're opting out of hot upgrades when deploying changes that include new database migration files. This is because our current automated migrations are fired off on start of the phoenix application, and hot upgrades don't issue a restart of the phoenix application (that's their chief benefit). However, using distillery hooks, we can still manage this, saving disconnects for users who are mid-retro when deploying a change that includes a database migration.

Acceptance Criteria:

• hot upgrade deploys automatically run migrations
• we no longer opt-out of hot upgrades when the deployed diff contains a new migration

Helpful Links:

• Distillery - Running Migrations
• Distillery - Boot Hooks