fix context cards for admins who can't read the course list

test plan:
 - enable student context cards feature
 - create an admin role that has "Users - view list" permission
   but no permissions on courses
 - as an admin in that role, go to /courses/X/users
 - click a student
 - the context card tray thing should appear
   (it might not have much in it, but it should appear)

fixes ADMIN-2207

Change-Id: I78d86dbe11e0d3ced3490027fd2f48e9da513625
Reviewed-on: https://gerrit.instructure.com/172592
Tested-by: Jenkins
Reviewed-by: Carl Kibler <ckibler@instructure.com>
QA-Review: Carl Kibler <ckibler@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>

app/jsx/context_cards/GraphQLStudentContextTray.js

@@ -95,13 +95,24 @@ const SCC_QUERY = gql`
   }
 `
 
+// for admins who can view the roster but not the course list
+function placeholderCourse(courseId) {
+  return {
+    _id: courseId,
+    permissions: {},
+    submissionsConnection: {
+      edges: []
+    }
+  }
+}
+
 export default props => {
   return (
     <ApolloProvider client={client}>
       <Query query={SCC_QUERY} variables={{courseId: props.courseId, studentId: props.studentId}}>
         {({data, loading}) => {
           const {course, user} = data
-          return <StudentContextTray data={{loading, course, user}} {...props} />
+          return <StudentContextTray data={{loading, course: course || placeholderCourse(props.courseId), user}} {...props} />
         }}
       </Query>
     </ApolloProvider>

app/jsx/context_cards/StudentContextTray.js

@@ -206,7 +206,7 @@ export default class StudentContextTray extends React.Component {
                 <div>
                   <header className="StudentContextTray-Header">
                     <Avatar user={user}
-                      canMasquerade={course.permissions.become_user}
+                      canMasquerade={course.permissions && course.permissions.become_user}
                       courseId={this.props.courseId}
                     />