Allow User.search to find by chef account username

Users should be able to add cookbook collaborators by searching for them
by chef account username. This adds a new has_one association called
chef_account then searches on that association using username.
Move add collaborators form into modal

This moves the add collaborators form into a modal which feels a bit
more fluid then going to another page as it provides a bit more context
for what you're adding collaborators to.
Merge pull request #306 from opscode/refine-adding-collaborators

Refine adding collaborators