Add a new language field language_name_display

[sandbergja]

User story

As a developer, after I run bundle exec rake orangetheses:cache_theses, I will receive a JSON file that includes both a language_facet and language_name_display key for each document, both with the same value.

Background

Previously, Orangelight used the same solr field, language_facet, for both its show page and its facets. pulibrary/bibdata#2069, however, requires different indexing logic for the show page vs the facets. pulibrary/bibdata#2092 introduced a new field specifically for the show page, called language_name_display.

So, the language_name_display generated by orangetheses will display on the orangelight show page, and the language_facet will continue to display in the orangelight facets.