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/mode_select.lua

@@ -47,9 +47,10 @@ function ModeSelectScene:new()
 		ruleset = current_ruleset,
 		select = "mode",
 	}
-	presence.details = "In menus"
-	presence.state = "Choosing a mode"
-	discordRPC.updatePresence(presence)
+	DiscordRPC:update({
+        details = "In menus",
+        state = "Choosing a mode",
+    })
 end
 
 function ModeSelectScene:update()