OAuth - add a little notes section for admins to remember details about external accounts

Summary: Fixes T4755. This also includes putting in a note that Google might ToS you to use the Google+ API. Lots of code here as there was some repeated stuff between OAuth1 and OAuth2 so I made a base OAuth with less-base OAuth1 and OAuth2 inheriting from it. The JIRA provider remains an independent mess and didn't get the notes field thing.

Test Plan: looked at providers and read pretty instructions.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4755

Differential Revision: https://secure.phabricator.com/D8726