Bug 8206: Specify index in OPAC authority search

Bug 8206: Specify index in OPAC authority search

Adds a dropdown to allow the user to choose to search the 'mainentry,'
and 'match-heading'indexes, in addition to the 'any' index which is
the default.

To test (you will need authority records with see-from and
see-also headings):
1. Go to the browse subjects and authors section in the OPAC
2. Do a search for a fairly generic term that is used as a
   see-also term in a lot of records. Note that your search
   results include both the record that you wanted and all
   the other records that refer to it.
3. Apply the patch.
4. Repeat the search from step 2. Notice there is no change
   to the results.
5. Repeat the search from step 2, but choose "in main entry"
   and observe that you are now only seeing authority records
   with the search entered in their main entry.
6. Repeat the search from step 2, but choose "in any heading"
   and observe that the results are the same as in step 2. This
   is intentional, so that in the future notes in authorities
   can be made searchable.
7. Search for an obsolete form of heading with the "in keyword"
   option selected. Notice what results you get.

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Works nicely. Hiding the new options can be done with CSS:

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>