escape chars in field name for qs queries

[albanm]

Some chars cannot be used in ES query_string queries. It seems that a backslash escape is supported, cf https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-query-string-query.html#_field_names.

If the user doesn't provide the backslash the ES error is returned "Expected "!", "&&", "(", "+", "-", ".", "/", "AND", "NOT", "OR", "[", """, "\", "{", "||", [^: \t\r\n\x0C{}()"+-/^~[]], end of input, or whitespace but ":" found.".

If the user provides the backslash an other error is returned "Impossible de faire une recherche sur le champ Gamme_de_prix_(valeur), il n'existe pas dans le jeu de données.".

• accept backslash in user queries
• try to implement auto-escaping for the simpler cases (FIELDNAME: value) by using our knowledge of all existing fields