Prompt users to attempt download of configured data with URL when no file is present

[carlhiggs]

Previously, it was requested by @jcelso that if a data source were configured with a URL, this could be automatically downloaded.

Automatic download from configuration files could potentially be risky if one had not authored these themselves, however, it would be useful to prompt a user with the configured URL and confirm with them that they wish to download this.

This has applications for producing reports in new languages to save users downloading new fonts that have been configured themselves. Instead (for #417) they should be prompted to check if they would like to download the configured font to provide support for a configured language.