More relevant search results

This pull request changes the default ranking algorithm used by Sphinx to make owned sentences appear first. My personal goal was to lower the visibility of the problematic sentences from the Takana Corpus, but I think Tatoeba as a whole can benefit from this, since unadopted sentences are likely to be less good than owned’s.

I added the user id and the creation date into the Sphinx indexes, which means we can use these informations while performing searches. Like filtering by user or sorting by creation date (though there is no interface for that yet).

**Warning:** @trang On the production server, you should regenerate the Sphinx config with the updated script, and refresh the reindexes, *before* pushing that code on production. This means you’d better manually copy the updated `generate_sphinx_conf.php` to the server prior to running `git pull`.