`bin/setup` and `bin/update` are currently almost the same file. The
only thing that keeps them apart is that one is running `bin/rails
db:setup` and the other `bin/rails db:migrate`.

I'm suggesting here that they should be a unique script, which needs to
be idempotent.

- New to a project, need to get started? `bin/setup`
- Need to install new dependencies that were added recently? `bin/setup`.

Before deprecating `bin/update`, I'm suggesting we just have it call
`bin/setup`.