Smooth out Phortune purchase completion flow
Summary:
Ref T2787. Currently, we dump the user back into the application. Instead, give them a confirmation screen and then let them continue.
Also fix a couple of unit tests I adjusted the underlying behavior of somewhat-recently in libphutil.
Test Plan: {F215498}
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T2787
Differential Revision: https://secure.phabricator.com/D10672