Create a BE endpoint to fetch the leaderboards

[starmarek]

• The structure of the answer fro the endpoint:

  {
  "categories": {
      "<name-of-category>": {
          "<name_of_the_user>": <number_of_answers>
      }
  }
  }

  Example:

  {
  "categories": {
      "History": {
          "dupa": 25
          "starmarek": 1,
      },
       "Science": {
          "starmarek": 26,
          "dupa": 0
      }
  }
  }

• Remember to order the users by the number of anaswers, so you don't have to order that in the FE (so that the leaderboard is already "prepared")