Add warnings to JIRA auth provider that we only support JIRA 6

Summary: Ref T4289. Make it clear that this provider does not currently work with JIRA 5.

Test Plan: Viewed JIRA provider from `/auth/`, saw warnings.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T4289

Differential Revision: https://secure.phabricator.com/D7906
Herald - tweak accepted differential revision check slightly

Summary: use the loadReviewedBy function, which seems to do what we want -- returns a reviewer IFF the last thing was an accept

Test Plan: i believe in the power of loadReviewedBy

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran, aarwine

Differential Revision: https://secure.phabricator.com/D7903