Prevent duplicate requests via the request entity add-form route

The approach I took here was to make a configurable option to toggle whether the entity reference selection handler should include users that are active reviewers. Users are considered "active" if they have an active review (i.e. a published review regardless of approval status).

The effect of this change is that the standalone request form does not include any users with an active review. On the entity workflow forms, they are included in the selection options, however, those forms in particular have additional logic which disables selection of active reviewers. This provides a visual indication that a review has already been requested from that user.

!6 got in, I added a merge commit, and solved related merge conflicts.

After that, I merged this in.

Automatically closed - issue fixed for 2 weeks with no activity.