Trim preview template so that createElement doesn't pick up an empty tex...

Fixed a bug I encountered where I was loading providing a preview template like the below:

    this.dropzone = new Dropzone(this.$el.find('form').get(0), {
      previewsContainer: this.$el.find('.dz-prev-container').get(0)
    });

    <script id="create-idea-dz-preview" type="text/template">
      <div class="dz-preview dz-file-preview">
        ...
      </div>
    </script>

Then when creating the `previewElement` from the template using `Dropzone.createElement` it returns the text node rather than the container div and then throws an error when it tries to access `querySelector`.