Add null check to getFeedbackResponseForStudent

Also added a test for the null check
Added log message to log null student when getting feedback response
Catch DatastoreTimeoutException in ControllerServlet
enhance Sanitizer.sanitizeForHtml() to filter forward slash

this is because, many XSS attacks use it to close the tag
sanitize for account
sanitize for comment
sanitize for course
sanitize for evaluation
update comment's sanitization
sanitize for fbQuestion
sanitize for frComment
sanitize for fbReponse
sanitize for fbSession
update Sanitizer.sanitizeForHtml(): remove filter for \n and \r
sanitize for instructor
sanitize for student
sanitize for submission
Move ReminderTests to the bottom of component tests
Merge branch 'bootstrap' into Issue2115
try to make dev green