Enable file downloads to go through a proxy if needed

[ThrawnCA]

Optionally use the ckan.download_proxy config value as a proxy when downloading files.

This allows the site owner to manage cases where file access depends on network location, or requires authentication.

[ThrawnCA]

@amercader Is there anything further that needs to be done before this is suitable to be merged? It's quite a small change, and entirely backwards compatible.

[amercader]

Thanks @ThrawnCA this is long overdue. I've merged your PRs in ckanext-xloader, datapusher and ckanext-validation (ckanext-archiver is pending: ckan/ckanext-archiver#78). Is there anything else I missed?

The one thing we would need is a PR is the main ckan repo that:

• Documents the ckan.downlaod_proxy option in https://github.com/ckan/ckan/blob/master/doc/maintaining/configuration.rst
• Adds a changes file to include an entry on the changelog describing the new option

As this is backwards compatible we can backport it to previous CKAN versions so it appears in their docs.

Many thanks for your work and your patience

[ThrawnCA]

@amercader I wasn't sure how to name the 'changes' file, since there isn't a core issue for this, but I've raised a pull request.