Show an error when entering the same value from the settings (#326)

Closes #324

TeamOctolings.Octobot/Data/Options/IGuildOption.cs

@@ -7,6 +7,7 @@ public interface IGuildOption
 {
     string Name { get; }
     string Display(JsonNode settings);
+    Result<bool> ValueEquals(JsonNode settings, string value);
     Result Set(JsonNode settings, string from);
     Result Reset(JsonNode settings);
 }