Fixes owner being allowed to demote himself (#834)

* Fixes owner demoting himself to subowner

This occurred whilst owner demoted himself to a subowner that led to non existence of the true owner

* Adds error on demote-yourself message

src/main/java/world/bentobox/bentobox/api/commands/island/team/IslandTeamPromoteCommand.java

@@ -52,6 +52,11 @@ public class IslandTeamPromoteCommand extends CompositeCommand {
             user.sendMessage("general.errors.unknown-player", TextVariables.NAME, args.get(0));
             return true;
         }
+        // Check if the user is not trying to promote/ demote himself
+        if (target == user) {
+            user.sendMessage("demote.errors.cant-demote-yourself");
+            return true;
+        }
         if (!inTeam(getWorld(), target) || !getOwner(getWorld(), user).equals(getOwner(getWorld(), target))) {
             user.sendMessage("general.errors.not-in-team");
             return true;

src/main/resources/locales/en-US.yml

@@ -507,6 +507,8 @@ commands:
       demote:
         description: "demote a player on your island down a rank"
         parameters: "<player>"
+        errors:
+          cant-demote-yourself: "&cYou can't demote yourself!"
         failure: "&cPlayer cannot be demoted any further!"
         success: "&aDemoted [name] to [rank]"      
       promote: