Improve the elasticsearch query handling

The query string for the elasticsearch now works a bit more like the google search:

• all single terms can now be found in different fields of the search index
• it is possible to quote two terms so that they need to be found together ("quoted text")
• we can add alternatives with OR (bla OR blub)
• it is possible to negate the search term when putting an '-' prefix before the term (-unwanted)