Snippets by namlook

A streamable "export to CSV" admin action

This snippets is inspired from [#2995](https://djangosnippets.org/snippets/2995/) but add the following: * get rid of `singledispatch` so we can use it with python2 without pip installing anything * streaming capabilities * the configuration params (header, fields, exclude...) are passed to the action function so we don't pollute the ModelAdmin class * fix utf8 issue in header