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/MessageBuilder.java

@@ -58,9 +58,12 @@ public class MessageBuilder {
     return this;
   }
 
-  public MessageBuilder replaceFromTo(Prestige prestige) {
-    replace(Variable.FROM, prestige.getFrom());
-    replace(Variable.TO, prestige.getTo());
+  public MessageBuilder replaceFromTo(Rank rank) {
+    if(rank instanceof Prestige) {
+      Prestige prestige = (Prestige) rank;
+      replace(Variable.FROM, prestige.getFrom());
+      replace(Variable.TO, prestige.getTo());
+    }
     return this;
   }
 

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);