Simplify setup of library CMS locally

[murny]

The current README suggests manually replacing the Rails.application.secrets.cms_user / Rails.application.secrets.cms_password everywhere in our app (with dev-username and dev-password), but this shouldn't be necessary. Can just use the values in secrets.yml instead.

Also, we force users to go to Admin panel, create a site, then hop into console and run a rake task. Could we simplify this? How about having rails db:seeds do all this work for us? It could simply create the site and invoke the seed task for us.

This would mean rails db:create && rails db:migrate and the entire setup of the site, could just become rails db:setup instead.