Add mute role support & fix /unmute (#109)

- Added support for `MuteRole`, now if you add any role to this setting,
then try to mute a member, all his roles will be removed except for the
one you set in this setting.
- Fixed `/unmute`, that tried to set target's display name to unmute
reason.

---------

Signed-off-by: Macintosh II <mctaylxrs@outlook.com>

src/Data/MemberData.cs

@@ -13,6 +13,7 @@ public sealed class MemberData
 
     public ulong Id { get; }
     public DateTimeOffset? BannedUntil { get; set; }
+    public DateTimeOffset? MutedUntil { get; set; }
     public List<ulong> Roles { get; set; } = new();
     public List<Reminder> Reminders { get; } = new();
 }