misc. changes (check description)

rename SECONDS to SECONDS_LEFT, and add SECONDS.
add %rankup_rank_money_left_<rank>% placeholder
add permission requirement

src/main/java/sh/okx/rankup/messages/Variable.java

@@ -17,7 +17,8 @@ public enum Variable {
   AMOUNT_NEEDED,
   PERCENT_DONE,
   PERCENT_LEFT,
-  SECONDS;
+  SECONDS,
+  SECONDS_LEFT;
 
   public String replace(String message, String value, String type) {
     Pattern pattern = Pattern.compile("\\{" + type + "_" + this + "}", Pattern.CASE_INSENSITIVE);