Fix error when executing `/leaderboard` without after but with before time

[TimJentzsch]

Relevant issue: Closes #141

Description:

The datetime that is used to calculate the duration of the leaderboard time when no after time was provided was timezone naive. This resulted in a timezone error when the duration was calculated and made the command fail. It now has UTC added to avoid this problem.

Checklist:

• [x] Code Quality
• [x] Pep-8
• [ ] Tests (if applicable)
• [x] Success Criteria Met
• [x] Inline Documentation
• [ ] Wiki Documentation (if applicable)