Reduce method complexity in /ban, /unban and some other commands (#50)

This PR does numerous things to reduce method complexity:
- Created an extension method
`FeedbackService#SendContextualEmbedResultAsync`, which directly takes a
`Result<Embed>` and checks it once in the extension method instead of
multiple times in all commands;
- Split the command methods for `/ban` and `/unban` into 2 parts:
`Execute(Un)Ban` and `(Un)BanUserAsync`. The former will check all the
needed results and will pass the outputs into the latter;
- Extracted the method for logging an action into Private- and
PublicFeedbackChannels. It now resides in UtilityService. Right now,
only `/ban` and `/unban` make use of that method;
- Created an extension method `Snowflake#EmptyOrEqualTo`, that combines
the task of checking if a Snowflake is empty and checking if that
Snowflake is equal to another Snowflake.

Similar changes will be made to other command groups in future PRs. This
is not done here to make the reviewing process easier.

Signed-off-by: Octol1ttle <l1ttleofficial@outlook.com>

src/Commands/KickCommandGroup.cs

@@ -83,10 +83,7 @@ public class KickCommandGroup : CommandGroup {
             var embed = new EmbedBuilder().WithSmallTitle(Messages.UserNotFoundShort, currentUser)
                 .WithColour(ColorsList.Red).Build();
 
-            if (!embed.IsDefined(out var alreadyBuilt))
-                return Result.FromError(embed);
-
-            return (Result)await _feedbackService.SendContextualEmbedAsync(alreadyBuilt, ct: CancellationToken);
+            return await _feedbackService.SendContextualEmbedResultAsync(embed, CancellationToken);
         }
 
         var interactionResult
@@ -162,9 +159,6 @@ public class KickCommandGroup : CommandGroup {
             }
         }
 
-        if (!responseEmbed.IsDefined(out var built))
-            return Result.FromError(responseEmbed);
-
-        return (Result)await _feedbackService.SendContextualEmbedAsync(built, ct: CancellationToken);
+        return await _feedbackService.SendContextualEmbedResultAsync(responseEmbed, CancellationToken);
     }
 }