Admin filter by taxon and geo_entity

This PR adds extra conditions in the `document_search.rb`file to allow searches by `TaxonConcept` and `GeoEntity` (restricted to `[GeoEntityType::COUNTRY, GeoEntityType::TERRITORY]`).

Two new select2 fields are added to the `documents/index.html.erb` template, and `documents.js.coffee` is completed with configuration for the geo entity select2 field.
I'm conscious that this may not be a viable solution with a big amount of data (although the pagination should avoid problems), so I committed it mainly to discuss about it. :confused: