Add 'ModeratorRole' guild setting (#290)

Octobot has various moderation commands such as /ban, /mute, /kick.
These commands add multiple features to Discord's built-in functions
(such as durations and logging). Some admins may want to force their
users to use Octobot's commands instead of Discord UI functions.
However, due to the current design, they can't take away the permissions
as that remove access to the respective command.

This PR adds the `ModeratorRole` option which allows anyone who has
`ManageMessages` permission and the role to perform any moderator
action.

If the role is not set, the Discord permissions are checked instead. If
the user doesn't have the role, but has the permission, they can still
run the command.

---------

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

src/Commands/SettingsCommandGroup.cs

@@ -51,6 +51,7 @@ public class SettingsCommandGroup : CommandGroup
         GuildSettings.EventNotificationChannel,
         GuildSettings.DefaultRole,
         GuildSettings.MuteRole,
+        GuildSettings.ModeratorRole,
         GuildSettings.EventNotificationRole,
         GuildSettings.EventEarlyNotificationOffset
     ];