Add a leaderboard for XP

[smyalygames]

The XP and system is just bare-bones, and there aren't really that many features with it.

What should be added is a leaderboard.

Requirements:

• Command that sends embed that shows the top 5-10 people on the leaderboard
  • This requires creating a function for the database to get the people with the most amount of XP/Levels

Nice to have features

• The ability to go through multiple pages in the embed with buttons to see further in the leaderboard