Allow cloning of existing chart

[munichrocker]

add an argument to dw_create_chart that clones an existing chart from the current user
(like the UI function)

[munichrocker]

Idea: Should basically be a combination of dw_retrieve_chart_metadata and dw_create_chart

[munichrocker]

An API endpoint for this now already exists: https://developer.datawrapper.de/reference/postchartsidcopy