Refactor sort/filter code once again

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

PluralKit.Core/Database/Views/DatabaseViewsExt.cs

@@ -0,0 +1,34 @@
+#nullable enable
+using System.Collections.Generic;
+using System.Text;
+using System.Threading.Tasks;
+
+using Dapper;
+
+namespace PluralKit.Core
+{
+    public static class DatabaseViewsExt
+    {
+        public static Task<IEnumerable<SystemFronter>> QueryCurrentFronters(this IPKConnection conn, int system) =>
+            conn.QueryAsync<SystemFronter>("select * from system_fronters where system = @system", new {system});
+
+        public static Task<IEnumerable<ListedMember>> QueryMemberList(this IPKConnection conn, int system, PrivacyLevel? privacyFilter = null, string? filter = null, bool includeDescriptionInNameFilter = false)
+        {
+            StringBuilder query = new StringBuilder("select * from member_list where system = @system");
+
+            if (privacyFilter != null)
+                query.Append($" and member_privacy = {(int) privacyFilter}");
+
+            if (filter != null)
+            {
+                static string Filter(string column) => $"position(lower(@filter) in lower(coalesce({column}, ''))) > 0"; 
+
+                query.Append($" and ({Filter("name")} or {Filter("display_name")}");
+                if (includeDescriptionInNameFilter) query.Append($" or {Filter("description")}");
+                query.Append(")");
+            }
+            
+            return conn.QueryAsync<ListedMember>(query.ToString(), new {system, filter});
+        }
+    }
+}