Reduced MAX_DESCRIPTION_LENGTH from 500 to 240, in preparation for upcoming arbitrary db reshape.

src/main/java/org/qortal/arbitrary/metadata/ArbitraryDataTransactionMetadata.java

@@ -23,7 +23,7 @@ public class ArbitraryDataTransactionMetadata extends ArbitraryDataMetadata {
     private String mimeType;
 
     private static int MAX_TITLE_LENGTH = 80;
-    private static int MAX_DESCRIPTION_LENGTH = 500;
+    private static int MAX_DESCRIPTION_LENGTH = 240;
     private static int MAX_TAG_LENGTH = 20;
     private static int MAX_TAGS_COUNT = 5;