Support "JIRA Issue URIs" as a Herald field for revisions

Summary:
See PHI1175. An install would like to trigger some reminders/guidance if users don't link revisions to JIRA issues.

Expose "JIRA Issue URIs" as a field so Herald can act on the presence or absence of issues.

I'm exposing "JIRA Issue URIs", not a field like "[ Has Jira Issue ][ is true ]", since it's a bit more flexible: you can use a regexp to test against particular PROJ-123 project prefixes in JIRA, for example.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20386