Searching should support sorting by classifications

[cdrini]

Although searches like https://dev.openlibrary.org/search?q=lcc%3AQA*&mode=everything are possible (on dev), it is currently not possible to view the results in "Shelf order". This data lives in solr, but has just not been exposed to any of the OL search apis.

This would be essential for a "Sort by shelf order" feature.

Summary of Requirements

• Add &sort=lcc parameter to /search and /search.json
• Add &sort=ddc parameter to /search and /search.json
• Bonus: When sorting by a classification, display the classification in the search results.
• Bonus: Include in UI as an option; (Shelf Order (DDC) | Shelf Order (LCC))
• Bonus: Add GA tracking to all sort options

Note: lcc and ddc are multiValued fields in solr; this makes them ineligible for use as a sorting field. The fields lcc_sort and ddc_sort were created for this purpose, but they had to be single valued (1 per work), and the longed such value was used. This could create a kind of confusing situation where one could back unexpected results. We might want to expose lcc_sort and ddc_sort directly, maybe? Or automatically switch lcc: to lcc_sort if we're sorting? But that will change the number of results :/ Exposing lcc_sort might be better for now.

Stakeholders

@seabelis @LeadSongDog

[LeadSongDog]

It strikes me that these sorts are not necessarily for browse-time. The classifications intrinsically need only to be considered when saving work records, a much less costly thing. A work record could link to prev- and next-LCClass, prev- and next-DDC.