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:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2787

Differential Revision: https://secure.phabricator.com/D10672