[views] implement json / jsonp export views (closes #1942658) Json export views are based on the same model as CSV export views. There are two distinct views : - *jsonexport* : direct conversion of the result set into json - *ejsonexport* : convert entities into json The JSONP parameter is named ``callback`` (same as on geonames, dbepdia and a lot of sites) An optional `_indent` request parameter can be passed to pretty print the results.