Formtools group actions

This PR create the ability to include group actions button. Below is an example of group actions DSL in formtools grid configurations:

```
grid: {
    groupActions: [
        {
            label: 'Assign RM',
            action: 'assign/rm'
        }
    ]    
}
```
For each of the group action defined, a router mapping is required in your application to serve the content for the URL defined in action. The action url will be prepend with host and model name. For example, the action URL above would be formatted as per below:

```/{admin route}/model/{model name}/assign/rm```

The content of the URL will be inserted into a boostrap modal. Below is an example of the content required.

```
<div class="modal-content">
    <form role="form" method="post" action="{{url}}">
        <div class="modal-header">
            <button type="button" class="close" data-dismiss="modal" aria-hidden="true">&times;</button>
            <h4 class="modal-title" id="myModalLabel">{{title}}</h4>
        </div>
        <div class="modal-body">
            <!-- content here -->
        </div>
        <div class="modal-footer">
            <button type="submit" class="btn btn-primary" data-linz-control="group-action-save">Save</button>
            <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
        </div>
    </form>
</div>
```

To-do:
- [x] Add group actions support to formtools plugin
- [x] Add group actions buttons to model index template
- [x] Group actions buttons should be hidden by default, unless a checkbox in one of the row on model index template is selected
- [x] Render content from URL defined in group action button action to modal
- [x] Configure modal to post form to URL defined in action