Allow a "suspend/activate" semantic on projects, to except them from non-forced builds.

Currently, if a project is acting up in known ways, there's no way to quiesce it until the problem is dealt with. On projects where continuum spams a list on purpose, this can be a sort of "false negative" and can cause people who can do nothing about the notification to start to stop paying attention.

It would be nice to allow a project to be in either an active or suspended state, with the option for a project admin (not a normal developer) to "suspend" the project, so that it will no longer be built. This is highly risky functionality, but is helpful during project build structure maintenance, for example, so that people aren't spammed with such false negative project notifications.

It would also be good to have this on a project group basis.