Extract system/member guild settings and refactor DB access

(also refactor MemberAvatar now that I'm here)

PluralKit.Core/Models/SystemGuildSettings.cs

@@ -0,0 +1,11 @@
+namespace PluralKit.Core
+{
+    public class SystemGuildSettings
+    {
+        public ulong Guild { get; }
+        public bool ProxyEnabled { get; } = true;
+
+        public AutoproxyMode AutoproxyMode { get; } = AutoproxyMode.Off;
+        public int? AutoproxyMember { get; }
+    }
+}