More rats, cheats and bugfixes

• Fixed upgrades that would use their base cost after restarting the app. Buying once would result in the correct amount being shown again. This could be exploited.
• Fixed bug where you could get stuck if you had the max number of rats but none to merge. This would lock the player out buying more rats and unable to let more spawn naturally.
• Added the final 20 Rats making the total of Rats 50. (Both Regular and Shiny)
• Updated the Index, so the player can unlock every single Index entry.
• Removed the Achievement Button.
• Added Cheats.