Add JIRA as an authentication provider

Summary:
Ref T3687. Depends on D6867. This allows login/registration through JIRA.

The notable difference between this and other providers is that we need to do configuration in two stages, since we need to generate and save a public/private keypair before we can give the user configuration instructions, which takes several seconds and can't change once we've told them to do it.

To this effect, the edit form renders two separate stages, a "setup" stage and a "configure" stage. In the setup stage the user identifies the install and provides the URL. They hit save, we generate a keypair, and take them to the configure stage. In the configure stage, they're walked through setting up all the keys. This ends up feeling a touch rough, but overall pretty reasonable, and we haven't lost much generality.

Test Plan: {F57059}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3687

Differential Revision: https://secure.phabricator.com/D6868