Media: Automatically select uploading attachments in single-select workflows.

The code now automatically selects uploading attachments in all workflows. In a workflow that selects multiple attachments, all attachments added to the upload queue are selected. In a workflow that selects a single attachment (e.g. selecting a featured image), the last attachment added to the upload queue is selected.

see #22817.
Media: Check that a router region exists before attempting to access its child views. Add relevant documentation. see #22930.