Convert autoproxy timeout to use seconds

PluralKit.Core/Database/Functions/MessageContext.cs

@@ -25,6 +25,6 @@ namespace PluralKit.Core
         public string? SystemTag { get; }
         public string? SystemAvatar { get; }
         public bool AllowAutoproxy { get; }
-        public int LatchTimeout { get; }
+        public int? LatchTimeout { get; }
     }
 }

PluralKit.Core/Database/Migrations/12.sql

@@ -1,9 +1,10 @@
--- SCHEMA VERSION 12: <insert date> --
+-- SCHEMA VERSION 12: 2020-12-08 --
 -- Add disabling front/latch autoproxy per-member --
 -- Add disabling autoproxy per-account --
 -- Add configurable latch timeout --
 
 alter table members add column allow_autoproxy bool not null default true;
 alter table accounts add column allow_autoproxy bool not null default true;
-alter table systems add column latch_timeout int not null default -1;
+alter table systems add column latch_timeout int; -- in seconds
+
 update info set schema_version = 12;