Add a new Build Trigger Type: "Manual" so that certain projects can be manually
started even if security is turned on.

Currently if security is turned on, no one except the administrator can trigger
a build manually.

I think this makes sense for certain types of projects. I have nightly,
continuous and so called "fast" builds. The fast builds are doing only a build
and deploy to get changes published in a short time. I would like to let users
trigger these projects manually in the web page. I think this differs from the
remote trigger which is explicitly designed for scripts.