Parse length to integer

When using the little up/down button we get strings instead of integers,
and those fail the rule validation. Fixes https://github.com/galaxyproject/galaxy/issues/6800

client/galaxy/scripts/mvc/rules/rule-definitions.js

@@ -297,7 +297,7 @@ const RULES = {
         },
         save: (component, rule) => {
             rule.target_column = component.addColumnSubstrTarget;
-            rule.length = component.addColumnSubstrLength;
+            rule.length = parseInt(component.addColumnSubstrLength);
             rule.substr_type = component.addColumnSubstrType;
         },
         apply: (rule, data, sources, columns) => {