Install MultipleWorkflowPlugin as workflow controller from admin page

[Cinc-th]

Admins must set the plugin as a workflow controller in trac.ini according to installation instructions.

If not done it's possible to define workflows using the admin page but they are not picked up when creating/modifying tickets.

Enhancement

It's easy to forget that installation step so show a warning on the admin page when the plugin isn't currently configured as a workflow controller.

Allow the user to set the plugin as a controller without manually changing trac.ini by providing a suitable interface on the admin page.

[Cinc-th]

In 17784:

MultipleWorkflowPlugin: let user install the plugin as a workflow controller from the admin page.

closes #13839