Discord RPC cleanup

- Loading Discord RPC is now handled by `load/rpc.lua`
- Removed `presence` global, call `DiscordRPC:update()` directly with what needs updating
- Game doesn't crash anymore if the Discord RPC fails to load
- Added RPC variables in the gamemode superclass to let each gamemode handle its special case

tetris/modes/gamemode.lua

@@ -40,6 +40,7 @@ function GameMode:new()
 	self.draw_section_times = false
 	self.draw_secondary_section_times = false
 	self.big_mode = false
+    self.rpc_details = "In game"
 	-- variables related to configurable parameters
 	self.drop_locked = false
 	self.hard_drop_locked = false