Refactor RDS handling

[adrian-golian]

Create a handler for RDS files, which will find, read, parse and validate a game state file, before returning a dataframe to whatever called for it. Anywhere game state files are being read, there should be error handling, to return a 500 "Game state was corrupted" in case of mishaps. Once there's a handler, we can think of mitigating issues, like repairing corrupted game states, or reading in the latest previous state.

[maciej-pomykala]

No longer relevant after #169