Allow merchants to disable payment providers and show more UI info

Summary:
Ref T2787.

• Allow merchants to disable payment providers.
• Show more useful information about providers on the payments page.
• Make test vs live more clear.
• Show merchant status.
• Add a description to merchants to flesh them out a bit -- the merchant areas of responsibilities seem to be fitting well with accounts, etc.

Test Plan:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2787

Differential Revision: https://secure.phabricator.com/D10662