Add a handlebars helper for variadic compound AND conditions.

e.g., from a comment in the commit:

// Execute the conditional code if all conditions are true.
// Example usage:
//     {{#if_and cond1 cond2 cond3}}
//         <p>All true</p>
//     {{/if_and}}

We'll use this for the email forwarding UI, but it may also be
generally useful, and easy to generalize to OR.

(imported from commit da601f94d9da300213ff46be50255135c014eca0)

static/js/templates.js

@@ -35,6 +35,22 @@ $(function () {
     Handlebars.registerHelper('plural', function (condition, one, other) {
         return (condition === 1) ? one : other;
     });
+
+    Handlebars.registerHelper('if_and', function () {
+        // Execute the conditional code if all conditions are true.
+        // Example usage:
+        //     {{#if_and cond1 cond2 cond3}}
+        //         <p>All true</p>
+        //     {{/if_and}}
+        var options = arguments[arguments.length - 1];
+        var i;
+        for (i = 0; i < arguments.length - 1; i++) {
+            if (!arguments[i]) {
+                return options.inverse(this);
+            }
+        }
+        return options.fn(this);
+    });
 });
 
 return exports;