landing page: switch previewed file not working

The [JS](https://github.com/inveniosoftware/cookiecutter-invenio-instance/blob/master/%7B%7Bcookiecutter.project_shortname%7D%7D/%7B%7Bcookiecutter.package_name%7D%7D/theme/assets/js/%7B%7Bcookiecutter.package_name%7D%7D/previewer.js#L3) points to a `div` with id `files` that was removed form here in order to use the `invenio-previwer` macros.

Something like this in `record.html` would fix it:

``` html
{% from 'invenio_previewer/macros.html' import file_list, preview_file %}

{% macro preview_file_box(file, pid) %}
{%- if file -%}

{% macro file_list_box(files, pid) %}

[....]

{%- set files = record.files %}
{{ preview_file_box(files|select_preview_file) }}
{{ file_list_box(files.dumps()) }}
```

Mainly note the *id*s in the outer div of each macro.

`dumps()` is needed due to https://github.com/inveniosoftware/invenio-records-files/issues/96

P.S: I think this is done https://github.com/inveniosoftware/cookiecutter-invenio-instance/issues/173 and should be closed?

Start with the referenced previewer.js line and the record.html template, then compare them with the invenio-previewer macros described in the issue. Check the status of issue 173 before starting, and verify that selecting a file switches the preview after the template changes.

Written by the indexing model from the issue text.