Convert legacy Advent Of Code command to slash command

[olivier-b-be]

Converts the last legacy command to slash command and removes legacy commands functionality

Resolves #244

Overview

Please bullet point the changes you have made below:

• Converted the legacy command to a slash command with two optional parameters
  • year: If a valid year is passed in it will use that year to either get the leaderboard of that year or the user statistics from that year
  • name: If a name is passed in that is found in the leaderboard ranking of a year it will return the user's statistics
• Slightly updated the visuals so that an anonymous user's id is also padded correctly in the leaderboard that is shown and added the year values for displaying info:

Before	After

• Removal of the legacy command handling way
  • Removed the legacy folders from src/commands and test/commands as the last command is now converted to a slash command
  • Deleted the CommandFactory and CommandParserHandler including the tests as they are no longer used for any (legacy) commands).
  • Moved the files in the commands/slash directory to the commands directory and updated the imports
• Updated README commands section to showcase the new way that commands are created for this bot

[olivier-b-be]

Incase you want a final check on the end product.