Move the upgrade purchase process into the app

There are two major obstacles in the upgrade purchase flow:

1. Finding the billing page
2. Completing the purchase process

This issue is to improve the second one—the purchasing process by exploring the possibility of moving it into the app itself. My rough estimation is that with removing both obstacles we could remove 80% of friction in the upgrade flow.

A similar approach is being explored via team-tasks#82 (closed) so let's stay up-to-date with developments there.