Add handling command ❌ reactions

and, uhh, rename invoker_id to author_id

PluralKit.Core/Database/Migrations/11.sql

@@ -4,7 +4,7 @@
 create table command_message
 (
 	message_id bigint primary key,
-	invoker_id bigint not null,
+	author_id bigint not null,
 	timestamp timestamp not null default now()
 );
 

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

@@ -9,7 +9,16 @@ 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)",
+			conn.QueryAsync("insert into command_message (message_id, author_id) values (@Message, @Author)",
 				new {Message = message_id, Author = author_id });
+
+		public Task<CommandMessage> GetCommandMessage(IPKConnection conn, ulong message_id) =>
+			conn.QuerySingleOrDefaultAsync<CommandMessage>("select message_id, author_id from command_message where message_id = @Message", 
+				new {Message = message_id});
+	}
+
+	public class CommandMessage
+	{
+		public ulong author_id { get; set; }
 	}
 }