Ability to sort search results by classification, random

[cdrini]

Closes #3574

Adds ability to sort by lcc/ddc to /search and /search.json

Technical

• Note we're using the ddc_sort/lcc_sort fields, because solr does not support sorting on multi-valued fields, which is what ddc and lcc are. This does mean that some things will appear out of order when doing e.g. ddc:8* sorted by ddc_sort. So I also exposed ddc_sort as a search field we can use -- although this has the caveat that some works will be missing.
• Random sorting from the UI will (for now) always use the same seed, so that results are in a consistent order as you paginate. Longer term we might want a way to use another seed, or maybe use a seed that's relative to today, so that the order changes daily? The URLs do let you do e.g. sort=random_my_seed to get a new random.

Testing

• DDC sorting works http://staging.openlibrary.org/search.json?q=ddc_sort%3A[801.0%20TO%20801.3]&mode=everything&fields=ddc,key&sort=ddc_sort%20asc
• Title sorting works Not really
• Random sorting works
• Normal sorting works for
  • relevance
  • editions
  • newest
  • oldest

Screenshot

Stakeholders

[mekarpeles]

lgtm, tested on staging v. prod