Make cookbook transfer more obvious

:convenience_store: 

This (hopefully) makes the process of transferring a cookbook to a new owner more intuitive.

Previously, there were 2 ways to transfer cookbook ownership:
* an admin could transfer the cookbook to anyone they wanted.
* the cookbook owner could transfer the cookbook to an existing collaborator by clicking on the collaborator's avatar on the cookbook details page.

Now, ownership transfer happens from the "Manage Cookbook" select menu on the right side of the page.  If the logged in user is an admin, things work as they did before.  They can select anyone to transfer ownership to, and it happens instantly.  Similarly, if the cookbook owner is logged in, and they search for a user to transfer ownership to, and that user is already a collaborator on the cookbook, then the transfer happens immediately.  

But, cookbook owners are welcome to search for any user they wish, whether they're existing collaborators or not.  If the user selected is not a collaborator, they'll receive an email informing them that someone wants to give them ownership of a cookbook and prompting them to accept or decline the invitation.  If they accept, they're the new owner.  If they decline (or fail to respond), nothing changes.

Closes https://github.com/opscode/supermarket/issues/825