Added support for system API/Team tokens

Since the Team and API tokens are normally the same across projects, I think it will be helpful to create system-wide Team/API token pairs and then have each job use one of those pairs.

This change introduces a global jelly file which provides a repeatable list of three items: Name, API Token, Team Token. The per-job configuration no longer provides fields for Team or API token, but has an option list of existing API/Team pairs.

I also added a small amount of backward-compatibility. The code will use existing API and Team tokens during a build. However, the next time any job is saved those values are lost and the build must use a system-wide configuration.