Bug 20589: Add field boosting and use query_string fields parameter

Bug 20589: Add field boosting and use query_string fields parameter

Generate a list of fields for the query_string query fields parameter,
with possible boosts, instead of using "_all"-field. Also add "search"
flag in search_marc_to_field table so that certain mappings can be
excluded from searches. Add option to include/exclude fields in
query_string "fields" parameter depending on searching in OPAC or staff
client. Refactor code to remove all other dependencies on "_all"-field.

How to test:
1) Reindex authorities and biblios.
2) Search biblios and try to verify that this works as expected.
3) Search authorities and try to verify that this works as expected.
4) Go to "Search engine configuration"
5) Change some "Boost", "Staff client", and "OPAC" settings and save.
6) Verify that those settings where saved accordingly.
7) Click the "Biblios" or "Authorities" tab and change one or more
   "Searchable" settings
8) Verfiy that those settings where saved accordingly.
9) Try to verify that these settings has taken effect by peforming
   some biblios and/or authorities searches.

Sponsorded-by: Gothenburg Univesity Library
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Alex Arnaud <alex.arnaud@biblibre.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>