Extract system/member guild settings and refactor DB access

(also refactor MemberAvatar now that I'm here)

PluralKit.Core/Models/MemberGuildSettings.cs

@@ -0,0 +1,19 @@
+#nullable enable
+namespace PluralKit.Core
+{
+    public class MemberGuildSettings
+    {
+        public int Member { get; }
+        public ulong Guild { get; }
+        public string? DisplayName { get; }
+        public string? AvatarUrl { get; }
+
+        public MemberGuildSettings() { }
+
+        public MemberGuildSettings(int member, ulong guild)
+        {
+            Member = member;
+            Guild = guild;
+        }
+    }
+}