Implement full-text searching

Index document content as well as the document metadata, allowing for documents and files to be found by their content. Matches on content should be weighted lower than matches on metadata.

This should be accompanied by performance tests as well.
I find it useful to add in extra information in these kind of log lines. For example:
```javascript
log().trace({'id': resource.id, 'previews': revision.previews}, 'No text to index');
```