OAuth Server -- add a doc and link to it in a few places

Summary: add a big ole HELP tab and make "scope" link to the specific
sub-section about scope

Test Plan:
read my doc a few times, it basically english
verified links looked correct and should work right once this is all in
production

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Maniphest Tasks: T910

Differential Revision: https://secure.phabricator.com/D1752