Fix KeyError when accessing feed, stats, games, and chat without logging in

[IsaacCheng9]

Problem Statement

• When accessing feed, stats (leaderboard, achievements), games (quizzes, flashcards), or chat without logging in, a KeyError: 'username' occurs.
  • This can be fixed by implementing the @login_required annotation with Flask, and hiding these items from the navigation bar when the user isn't logged in.
    • It may be better to hide the navigation bar entirely when the user isn't logged in.