behaviors: add loop control selector

Right now, our loop config in behaviours is purely based on the loop
limit. We can be smarter than that, though - sometimes we can know
when it's safe to *stop* clicking the same element over and over by
running another selector.

A good example of this is page pagination. Most sites with
JavaScript-based pagination will also disable the "next" button once
the browser has reached the last page of results. Using that, we can
configure a behaviour that looks something like this:

```yaml
- url_regex: '^.*$'
  behavior_js_template: umbraBehavior.js.j2
  request_idle_timeout_sec: 10
  default_parameters:
    interval: 10
    actions:
      - selector: .next
        repeatSameElement: true
        repeatUntilSelector: .next[disabled]
        # the repeatUntil selector makes long loops safe
        # since we'll break out of the loop once we hit the
        # disabled button!
        limit: 1000
```

Author: mistydemeo

brozzler/js-templates/umbraBehavior.js.j2

@@ -35,6 +35,7 @@ class UmbraBehavior {
         var selector = this.actions[k].selector;
         var childSelector = this.actions[k].childSelector;
         var repeatSameElement = this.actions[k].repeatSameElement ? this.actions[k].repeatSameElement : false;
+        var repeatUntilSelector = this.actions[k].repeatUntilSelector ? this.actions[k].repeatUntilSelector : null;
         var limit = this.actions[k].limit ? this.actions[k].limit : false;
         var action = this.actions[k].do ? this.actions[k].do : 'click';
         var rmSelector = this.actions[k].rmSelector ? this.actions[k].rmSelector : null;
@@ -93,8 +94,15 @@ class UmbraBehavior {
             }
 
             for ( var i = 0; i < repeats; i++) {
-                if (!repeatSameElement && this.alreadyDone.indexOf(doTargets[i]) > -1) {
-                    continue;
+                if (this.alreadyDone.indexOf(doTargets[i]) > -1) {
+                    if (!repeatSameElement) {
+                        continue;
+                    }
+
+                    // We were looping the same element but we're done now
+                    if (repeatSameElement && repeatUntilSelector && document.querySelector(repeatUntilSelector).length > 0) {
+                        break;
+                    }
                 }
                 if (!this.isVisible(doTargets[i])) {
                     continue;