Allow subscribers on Pipeline release systems to be edited

Summary: This allows the subscribers on a release system to be edited, so that we can add mailing lists and projects to the list of subscribers.

Test Plan: N/A

Reviewers: waynea, #koi, lucasr

Reviewed By: #koi, lucasr

JIRA Issues: KOI-804

Differential Revision: https://code.pageuppeople.com/D3365

Risk Rating: 1