Summary:
Ref T2787. Instead of making providers global configuration, make them a thing on merchants with web configuration.
Payment methods and some of the pyament workflow needs to be retooled a bit after this, but this seemed like a reasonable cutoff point for this diff.
Test Plan: See screenshots.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T2787
Differential Revision: https://secure.phabricator.com/D10649
