Add template for join as billing admin view
add utility for determining if a given user is an accounting admin
add is_billing_admin attribute to request
check to see if the current user is a billing admin. if they are not and are an accounting admin, show message
add view to domains to join as a Billing Account Admin (available to accounting users)
use is_accounting_admin utility
don't show the add credit or make payment buttons unless the current user is a billing admin (even if they are an ops user / superuser)
Merge pull request #3904 from dimagi/billing-admin-accounting

Allow Accounting Admins to join accounts as Billing Admin