[search] Support nested document and faceting

Records are already displayed properly by default

1. Facets http://yonik.com/solr-nested-objects/ (like a facet but with parent/child checkbox + parent filter)
2. Block joins http://yonik.com/solr-4-8-features/ (similar to multi query boxes, get parent/child + extra filter box)
3. https://medium.com/@alisazhila/solr-s-nesting-on-solr-s-capabilities-to-handle-deeply-nested-document-structures-50eeaaa4347a#.ox6bzafxp

Done:

1. Allow to define 1 similar level or tree
   1. Level1, Level2, Level3
   2. Level1, [Leve2A, Level2A1] / Level2B, Level3
2. Expand nested objects in grid result
3. Facets on
   1. parent
   2. children

Todo:

1. escape childDocuments in result
2. Make progress on https://issues.apache.org/jira/browse/SOLR-7672 and implement in Hue
   1. Autocomplete
3. Add nested objects to download
4. Much simpler Facet UX
5. readonly in grid result
6. Add example
7. Nested fields in grid/field lists?
8. Add distribution / graphing?

Follow-up

1. Indexing HUE-2961
2. https://issues.apache.org/jira/browse/SOLR-8998 Rollups