Try to add drop down on issue details to assign users.

Broken.
Improve the look and behavior of the assigned user selector.
Fix error in github api docs.  (wrong doc link)
Load project collaborators into repo model.
Add code to show collaborators and allow selecting them.
Refactor github api to use assignee API.
feature: Assigned user can be changed from detail view.
Put dropdown no toggle in it's own module.