app-sf-49: Add leaderboard in overview

[pan93412]

• feat(entity): Login Event
• feat(admin): LoginEvent
• feat(subscriber): Write login event when logging in
• feat(profile): Show login events count in profile
• feat(questions): Implement pass rate
• refactor(questions): Move pass rate logic to Question
• feat(challenge): Show pass rate
• feat(app): De-/Initialize tooltips for page navigation
• feat(challenge): Implement details of pass rate
• fix(comments): Set min=0 for comments count
• feat(overview): Grid layout & Leaderboard
• refactor: RWD optimization
• refactor(overview): Tweak gutter
• feat(overview): Implement Leaderboard

Fixed #49
Iterate from #55