added auto-payment logic, continued fiddling with update billing info button

- still getting error on stripe_card_token for dashboards#update_billing
- added process_auto_payment and auto_confirm_apprenticeship to
app_signup.rb (needs testing)
- Added rake task for scheduling auto-confirmation (needs testing)
- updated emails in app_signup.rb for auto payments, and added links to
cancel in the accept email
- filled in link to “Update my billing info” on app_signup confirm and
apprenticeship payment tabs
- Added explanation text to dashboard billing page