Expose content fields

Include additional fields
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Currently, not all fields available in the Elasticsearch index are exposed through the REST API. Some for good reasons (such as the `combined_index_data` field, which is purely stored for backup-and-restore purposes, and not interesting for users), whereas others might be useful for users interested in **all** the textual content of a record, for instance as a source for data analysis.

This pull requests enables the API user to request the inclusion of additional fields (`all_text`, which contains all the text used for search in the search endpoints, or `source_data`, which is the original content representation of the record).