[bug 904554] Add boosts for document queries

This re-adds the boosts for document queries (document_summary, document_content, ...).

You can verify the boosts are there by adding:

```
import logging
logging.getLogger('pyelasticsearch').setLevel(logging.DEBUG)
```

to the top of the ``search`` view in ``kitsune/search/views.py``. The output will show the complete ES search as well as the results. The ES search will show all the clauses and you can verify that the document ones have boosts.

r?