[mappings] Add stopword removal to indexed text fields

[alpgarcia]

To improve searches and visualization of most used words it would be needed to remove a set of stopwords. Imho, the easiest way to do it is using ElasticSearch features from index mappings:

https://www.elastic.co/guide/en/elasticsearch/reference/5.4/analysis-stop-tokenfilter.html https://www.elastic.co/guide/en/elasticsearch/guide/current/using-stopwords.html