Attach PDFs to Sponsor Invoice posts

[iandunn]

Currently copies of sponsor invoices from Quickbooks are just attached to the notification email that is sent to the organizing team, but it'd be better to attach them to their corresponding Sponsor Invoice post on WordCamp.org.

• Add a random key to the filename, to mitigate exposing e-mail addresses to someone crawling the uploads directory
• Upload PDF to media library, attach to corresponding sponsor invoice post
• Add link to PDF inside the invoice post, in the Status meta box
• Remove the attachment from the email and link to the copy on WordCamp.org instead

​https://wordpress.slack.com/archives/events/p1463097083001274
​https://wordpress.slack.com/archives/events/p1463156693001327

[dd32]

This ticket has been moved to GitHub ​https://github.com/WordPress/wordcamp.org/issues/586