Sponsor notification when sponsorship is received

[timiwahalahti]

Sponsors do not currently get any notification unless the WordCamp organizers manually send an email. Let's fix that by sending an automated email, looking like it came from the WordCamp organising team, to the sponsor when the invoice has been paid. This way, we help facilitate more communication between the organizers and sponsors.

Added a new function for this instead of refactoring existing notifications and adding sponsor as a CC to the same email that organisers do get. Their email contains internal titles and links, which are helpful for them but not for the sponsor's eyes.

Fixes #616

Props @angelasjin @iandunn @coreymckrill @andreamiddleton

Screenshots

How to test the changes in this Pull Request:

1. Create new sponsor invoice on WordCamp site
2. Send it on network payments
3. Mark it as paid on QBO
4. Run cron event wcbdsi_check_for_paid_invoices on your site
5. Check email (on dev mailcatcher)