Refactor sort/filter code once again

Now we handle sorting on the bot side, but still filter in the database

PluralKit.Core/Database/Views/SystemFronter.cs

@@ -0,0 +1,14 @@
+using NodaTime;
+
+namespace PluralKit.Core
+{
+    public class SystemFronter
+    {
+        public int SystemId { get; }
+        public int SwitchId { get; }
+        public Instant SwitchTimestamp { get; }
+        public int MemberId { get; }
+        public string MemberHid { get; }
+        public string MemberName { get; }
+    }
+}