Reinstate photo attribution on MP pages

Closes https://github.com/mysociety/theyworkforyou/issues/594.

![screen shot 2014-07-21 at 12 18 17](https://cloud.githubusercontent.com/assets/739624/3642852/cf82fdfa-10c8-11e4-9744-e55d8a994c78.png)

I couldn't work out how to get the photo attribution text and links for a given member, so I've stubbed it out temporarily:

```php
// Replace this with some code that actually
// checks for a photo attribution in $MEMBER.
$data['photo_attribution_text'] = 'Crown copyright, used under the Open Government Licence';
$data['photo_attribution_link'] = 'https://www.mysociety.org/horse';
```

This is good to go as soon as a developer has time to get the actual photo attribution text out of the model for a given person.