convert paying_subscription to payment_plan

also add failed signup cuke coverage
Merge branch 'master' of github.com:loomio/loomio into bug/payment-fields

Conflicts:
	extras/setup_group.rb
	features/step_definitions/start_new_group_steps.rb
Merge pull request #957 from loomio/bug/payment-fields

convert paying_subscription to payment_plan