[GH-497] Add Upgrade config import

[lotuuu]

Motivation

Adds configuration for Upgrades and their dependencies. They have been made in two different files because it's easier for development. It also seems like the tidier way to do it, since otherwise it could become kind of messy and difficult to track.
Closes lambdaclass/afk_gacha_game#497

Summary of changes

• Add upgrades.json and upgrade_dependencies.json.
• Add import functions for them.
• Tidy up the Config module that was needing some love.
  • Move all private functions to the end of the module.
  • Add doc to missing functions.
• Remove wrong fields in Buffs

How to test it?

• Run the app and check that everything runs normally.
• Tests too.
• Go through the steps in [GH-428] Implement Buffs for Dungeon #632 and check that everything still works.
• Inspect the uploaded data (upgrades, buffs, dependencies) and check that everything is there.