Add error messaging to course team page

Hitting "Add User" with no email address, or trying to add a user to the course
team multiple times
Add confirmation prompt when deleting a user from a course team
Studio: revises prompt messages for team roles UI
Stop using authz functions to change user groups

They contain too much incorrect permission checking
Add lettuce test for transferring ownership of a course using the course team page
Lettuce: confirm deletion prompts on course team page
Create a dedicated CMS.Models.Course Backbone model

Initialize on every page that has a context_course set
Abstract out the unknown error message and i18n
Curly quotes around course name
Add a lettuce test for removing own admin permission on course team
Merge pull request #538 from edx/db/course-team-admin-grants

Add error messaging to course team page