Make db:setup a proper rake task

Move it into a `.rake` file and give it a description so it shows up in
`rake -T`.
Add db:migrate rake task

For consistency with all the other apps, add a rake task to run database
migrations which can be run as part of deploys.

Log all output to STDOUT so it can be seen in the deploy logs in
Jenkins.
Update local Postgres config

There is a vagrant user created for Postgres on the development VM. The
email_alert_api user was only ever manually created.

This supercedes eb53232 which was accidentally blatted in a rebase by
3793cdc.
Remove obsolete migrate script

We have a db:migrate rake task so this isn't needed any more.
Merge pull request #29 from alphagov/add-db-migrate-rake-task

Add db migrate rake task