Add show history button to Workflow Tasks

Story: https://reciprocitylabs.atlassian.net/browse/CORE-6
b/17062748
Not by default; Control types fix their event handlers in the prototype, so it's usually defined by type.  However, you can use a similar pattern as shown in dashboard_controller.js lines 39-54, where you can pass a maybe-function in the control instance options (content_controller_options in this case), and call it if it exists from a generic handler, so let's say we did something like this.

```
"button[custom-event] click" : function(el, ev) {
  var event_name = el.attr("custom-event");
  if(this.options.events && typeof this.options.events[event_name] === "function") {
    this.options.events[event_name].apply(this, arguments);
  }
}
```

Then in our custom controller options:

```
events : {
  "show-history" : function(el, ev) {
    this.options.attr("mapping", el.attr("mapping"));
    this.reload_list();
  }
}
```