Cleans permissions, and reduces redundancy in permission checking code

src/main/resources/plugin.yml

@@ -99,7 +99,7 @@ permissions:
       stargate.option.nonetwork: true
       stargate.option.random: true
       stargate.option.quiet: true
-      stargate.option.nosign: true
+      stargate.option.invisible: true
   stargate.option.hidden:
     description: Allows the creation of a hidden stargate
     default: false
@@ -127,7 +127,7 @@ permissions:
   stargate.option.quiet:
     description: Allows the creation of a stargate which does not output anything to the chat
     default: false
-  stargate.option.nosign:
+  stargate.option.invisible:
     description: Allows the creation of a stargate which has no sign
     default: false
   stargate.admin.hidden: