Improved Character Selection UI

[cdsupina]

Changed character selection ui to use nested Bevy ui nodes instead of static images. Make sure to get the latest assets from aws with: python asset_manager.py --profile thetawavedev-p download --s3-location free_assets --no-dryrun

• Players join that same as before by pressing the join input on their input device
• Carousel showing 3 characters at a time to select character
• Name, abilities, and stats update when different characters are selected
  • Each stat has a unique formula for determining the characters' effectiveness in that stat (Health, Damage, Speed, Cooldowns/Fire Rate, Range, Size)
• After selecting their character, players can "ready up", when all characters are ready the app state advances to InitializeRun
• Ui currently supports up to four players, but the maximum players that can join can be set with the max_players value in game_parameters.ron