Added .imageonlychannel / .imageonly to prevent users from posting anything but images in the channel

src/NadekoBot/Db/Models/ImageOnlyChannel.cs

@@ -0,0 +1,8 @@
+namespace NadekoBot.Services.Database.Models
+{
+    public class ImageOnlyChannel : DbEntity
+    {
+        public ulong GuildId { get; set; }
+        public ulong ChannelId { get; set; }
+    }
+}