More maintainer fixes

:fork_and_knife: 

This corrects a few bugs that cropped up with collaborators.  This addresses the following Trello cards:

https://trello.com/c/UN4xLTmj/4-cookbook-maintainers-can-add-and-remove-cookbook-contributors-from-the-cookbook-show-view
https://trello.com/c/hERJ8aAK/8-cookbook-maintainers-can-transfer-ownership-to-other-contributors