Add co-author feature to bookshelf

[mmcky]

Add a co-author feature to support multi-author notebooks and provide credit.

This will be application based using userid and avatars with dynamic search of users in the system. It will not use an email system.

[Flintro]

Hey Matt, just fyi from what I can tell the coauthor feature has been implemented on the page but doesn't actually work at the moment.

[tlyon3]

Because this hasn't been implemented fully yet, I went ahead and hid the fields in commit 0aaca9c8302944cb552077fe884772853d75423d

I'll unhide them once it's implemented