Add year select (dropdown) to past datathon leaderboard (#430)

[jang-35]

Description

Added year select (dropdown) to past datathon leaderboard.

Implements #430

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• This change requires a documentation update
• Chore (anything else)

How Has This Been Tested?

• Responsiveness on various devices
• Desktop Browser
  • Apple Safari
  • Google Chrome
  • Microsoft Edge
  • Mozilla FireFox
• Mobile Browser
  • Apple Safari
  • Google Chrome
  • Microsoft Edge
  • Mozilla FireFox
• Other Tests:

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation

PR Closure

• Use Squash And Merge to Close Pull Request
• Delete any branch after merging

[ThunderRoar]

The leaderboard selection looks great!