final tasks before beta deployment

[bxie]

• [x] improved handling of placeholder hints
• [x] see if can make feedback work #373
• [x] improve Python feedback
• [x] provide practice or explanation of memory tables UI
• [x] remove follow-up questions
• [x] add tutorial #332
• [x] remove progress bar if no exercises (how code runs)
• [x] side nav: remove close button for C2 condition
• [x] update content to reflection Python 3 handling of errors
  • [x] digit processing
  • [x] arith operators
  • [x] data types
• [x] onboarding video
• [x] improved sign-posting for navigation (unicode arrows)
• [x] remove/hide follow-up questions
• [x] placeholder text for memory table UI
• [x] format traceback feedback for write code