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

scene/input_config.lua

@@ -23,9 +23,10 @@ function ConfigScene:new()
 	self.config = config.input
 	self.input_state = 1
 
-	presence.details = "In menus"
-	presence.state = "Changing input config"
-	discordRPC.updatePresence(presence)
+	DiscordRPC:update({
+        details = "In menus",
+        state = "Changing input config",
+    })
 end
 
 function ConfigScene:update()