[clarity/consistency] rename /explorev2/ -> /explore/ (#2802)

* rename /explorev2/ -> /explore/

* add redirect for existing explorev2 urls

* fix long line

* remove extra line

* fix missed ref in spec

superset/assets/javascripts/explore/validators.js

@@ -0,0 +1,32 @@
+/* Reusable validator functions used in controls definitions
+ *
+ * validator functions receive the v and the configuration of the control
+ * as arguments and return something that evals to false if v is valid,
+ * and an error message if not valid.
+ * */
+
+export function numeric(v) {
+  if (v && isNaN(v)) {
+    return 'is expected to be a number';
+  }
+  return false;
+}
+
+export function integer(v) {
+  if (v && (isNaN(v) || parseInt(v, 10) !== +(v))) {
+    return 'is expected to be an integer';
+  }
+  return false;
+}
+
+export function nonEmpty(v) {
+  if (
+      v === null ||
+      v === undefined ||
+      v === '' ||
+      (Array.isArray(v) && v.length === 0)
+  ) {
+    return 'cannot be empty';
+  }
+  return false;
+}