- Add storing bot response messages in postgres

- Add scheduled task to clean up said store

PluralKit.Core/Database/Repository/ModelRepository.CommandMessage.cs

@@ -0,0 +1,15 @@
+using System.Collections.Generic;
+using System.Data;
+using System.Threading.Tasks;
+
+using Dapper;
+
+namespace PluralKit.Core
+{
+    public partial class ModelRepository
+	{
+		public Task SaveCommandMessage(IPKConnection conn, ulong message_id, ulong author_id) =>
+			conn.QueryAsync("insert into command_message (message_id, invoker_id) values (@Message, @Author)",
+				new {Message = message_id, Author = author_id });
+	}
+}