Openlibrary: Enable searching for a classification on prod

Works on similar subjects normally are in similar-valued lc_classifications and/or dewey_decimal_class. It should be possible for clients to search on these fields to find such works.

Describe the problem that you'd like solved

1. Ensure the above-mentioned classifications are put in the SOLR index, not just the edition json
2. Add "Classification" to the list of fields for Advanced search at https://openlibrary.org/advancedsearch
3. Amend the search query to support such search.
4. Result should be a list of the matching works, not specific editions.

Proposal & Constraints

Nearly every major online catalogue is searchable by classification: LC, BnF, BL, etc.

1. Although an easy-to-use "search nearby" link would be great to have, it is a next-stage feature. Let's walk before running.
2. If client searches for QA135 they will expect to see a superset of the results found for QA135.5

Additional context

Doing this should help facilitate finding mergeable works, translations, etc. by the same author.

Stakeholders