WL-99 send a copy of the invoice to the finance@edx.org when registration codes are generated

changed the finance_email in the common.py file

added the test for the FiNANCE_EMAIL settings
Merge pull request #5982 from edx/muhhshoaib/WL-99-Invoicing-email-finance

WL-99 send a copy of the invoice to the finance@edx.org when registration codes are generated