Allow adding outside collaborators to a product#584

There are situations where I want somebody else to be able to modify one product, but don’t want to add them to the team (and grant access to all products by default). That’s why I’d suggest to add an option to add “Outside Collaborators” (Collaborators that aren’t part of the “Team” as specified in the account settings) to a product using the email adress of a registered Gumroad account. This could be done in the “Team” tab that I’ve described in #582 and #583
Check the attached mockup for more details

I hope they improve on this, especially split payouts per product when there’s collaboraiton would be amazing