Rebuilds: Allow admin to pause admin-initiated rebuilds

[chrisvire]

It is possible that something goes wrong with rebuilds. The admin should be able to pause new builds of the admin-initiated builds.

[FyreByrd]

Clarifying questions:

1. Should this be available for both SuperAdmins and OrganizationAdmins, or just SuperAdmins?
2. Should affected builds be paused in BuildEngine/CodeBuild? Can we even handle that?