Raw fields should be considered as text/keyword

[dadoonet]

As elasticsearch will by default to automatically guess the type, you could end up having conflicts between
metadata raw fields: a field which is first detected as a date but is getting for another document a value like
"in the seventies". In such a case, you could imagine forcing the mapping or defining an index mapping template.

This commit provides now a dynamic template for meta.raw.* fields:

      "dynamic_templates": [
        {
          "raw_as_text": {
            "path_match": "meta.raw.*",
            "mapping": {
              "type": "text",
              "fields": {
                "keyword": {
                  "type": "keyword",
                  "ignore_above": 256
                }
              }
            }
          }
        }
      ],

Closes #439.