JENKINS-12093 - Allow per-project & category throttling on the same job

Hi,

Please review and provide feedback.
Updated to always throttle on project settings, then optionally throttle by category.
Per existing behaviour, if project settings are 0, then no throttling occurs by project.

Other options could be
1. add new radio box to specify throttle by both project and category.
2. rework UI to have checkboxes to enable 'by project' and 'by category' independently so you can select both.

thanks
Geoff