Switch database to postgres

[MatMoore]

Currently we're using the rails default of sqlite.

We should switch to postgres. :elephant:

More info:

• http://howfuckedismydatabase.com/sqlite/
• http://guides.rubyonrails.org/active_record_postgresql.html
• http://guides.rubyonrails.org/configuring.html#configuring-a-postgresql-database

---

Things to do:

• [ ] Document how to install a postgres server for development
• [ ] Change the development configuration
• [ ] Document anything else you need to do when setting up the project
• [ ] Fix anything that breaks