Do NOT enable all roles for Transitions, upon enabling Workflow UI module.

This is a follow-up of #2228317: hook_user_role_insert is not called when Admin UI disabled..
The permission 'Participate in workflow' is now in the main module.
Testing also reveals the cause of an itch: "I thought I had disabled that role for workflow transitions."
It turned out that disabling and re-enabling Admin UI module (for performance reasons), caused all roles to be explicitely enabled for transitions. This should not happen! Espacially since some big site experience WSOD on the transitions page, due to the meny roles they have.

As of now, enabling Workflow Admin UI does not enable ALL roles for transitions.
I have left the upgrade hook 7001.
New roles are still by default enabled, but that should not be necessary.

I've added a link to the permissions page in the help text on the transitions page for easy referencing and linking.