Add seed data for topics, one content and course

Also add new seed infrastructure:
  on rake db:seed different seed files are loaded based on the environment set.
  db/seeds/all.rb is always loaded, then the <environment>.rb is loaded
Add Paymill support using the paymill/paymill-ruby gem

Currently we're using the PayButton, see https://www.paymill.com/en-gb/documentation-3/add-ons/paybutton/
Might need to switch to using our own form for aesthetic reasons :)
Merge branch 'master' into nf/paymill

Conflicts:
	app/controllers/payments_controller.rb
	app/views/bootcamps/show.html.erb
Add custom credit card form for paymill payments
Add HTML5 validations for paymill form
Disable Paymill form for now
Merge remote-tracking branch 'origin/master'

Conflicts:
	app/models/payment.rb
	app/views/courses/checkout.html.erb
	db/schema.rb