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.
As @mjankowski pointed out, seeds are a separate concept from demo data. Seeds are data that need to be populated in the database for the app to run correctly. For example, populating a `countries` table with the nations of the world. As such, I think seeds should be the same for all environments.

Demo data such as sample admins and users don't belong in seeds. They make more sense in a separate task such as `dev:prime` or `demo_data:reset`