[core] Massive setting system update

- Get rid of attribute crap. Now only interface is required
- Rename canUse to isAdmin in `Bot.IsUserAdminAsync` to match the actual meaning of variable
- Made `AnswerQuery` reusable by moving it from ContentStore to Core's CommonHandler
- Chat Settings edit frontend (inline keyboard)
- Rename NsfwOption to AllowNsfw and move it to ContentStore module
- Remove option backend testing commands
- Add KarmaMode option as an example of Select option type

Core/Data/ChatOptionValue.cs

@@ -1,6 +1,4 @@
-#nullable enable
-
-using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore;
 using System.Text.Json.Serialization;
 using System.Text.Json;
 using System;
@@ -15,7 +13,7 @@ public class ChatOptionValue
     public byte[] Value { get; set; }
 
 
-    public T GetValue<T>(T? defVal = default)
+    public T GetValue<T>(T defVal = default)
     {
         try
         {