Handle `dev:prime` seeds in convetional file

Rails' convention is to do DB seeding in `db/seeds.rb`. This moves the logic
from the `.rake` file to the `.rb` file.

It has the nice side-effect of being called via `rake db:seed`, in addition to
`rake dev:prime`.

Also, is allows for calls to `FactoryGirl` to be fully out-dented and not
crammed inside of nested blocks.
We've been really strict about always adding things to seeds instead of adding via the console over our past few projects. If you need to check something in development, create what you need and run the dev seeds from scratch and then put the new seeds in your PR. It builds up a standard setup so that anyone new on the project can immediately get in and play around with the app. It is also really useful for designers who need to be able to test interactions with different types of data.