Channel-picker: Show both user selectable channels and already-selected channels

This PR does several things to improve the channel picker UI.

It used to be that ONLY the channels of the user editing the media would show up in the channel picker.  That means that if a different user had a different set of channels, some selected channels might not show up.

This could have been problematic if when the media was saved, it was removed from unselected channels, however, I don't think that was happening (another UI problem as that action would be expected).

This PR aims to fix that potential problem by listing all currently selected channels in combination with user-selectable channels.   Currently selected channels that the user does not have permission for will be listed as read-only.

This also fixes a problem where in this 4.0_template branch, currently selected channels were not being selected in the UI.