fix(review): don't crash if channels and review users are missing

This is just a partial fix to #662 (closed). I don't yet know why chat channels are missing from the manuscript, or why the manuscript's review objects are missing their user. All I'm doing here is allowing the page to display without crashing if these are missing. The chat pane will be blank and the review fields will not be shown.

I intend to keep #662 (closed) open until I have understood the root cause.