[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.