Reviewer flow fixes

Feedback for issues #239 (closed), #306 (closed), #238 (closed). Issue #337 (closed)

• render item in learner mode initially for reviewer
• persist answers when reviewer tries to submit a question
• send reason for rejecting a review invitation to editor via email
• show red banner when invitation revoked or rejected
• when there are pending invitations or reviews, show warning to editor before "moving to production"
• notify reviewer via email when editor revokes invitation
• modify email template for review invitation