Add back report_uri CSP directive until Reporting API stabilises

[oplik0]

While the report_uri directive is deprecated, currently Report-To header required for report-to directive was removed from spec before it even became a Candidate Recommendation. Instead, current ED of Reporting API has a Reporting-Endpoints header that doesn't use the JSON values like the previous one, instead opting for a simpler groupname="url" syntax (with ability to add multiple comma-separated entries).
Considering that according to caniuse only about 70% of users have support for Report-To header, while over 93% support report-uri directive, for now report-uri seems to be more widely supported and could even live longer than Report-To header.

It can be added as a custom directive, but I think it'd be a good idea to re-add it as a normal directive. report-to CSP directive is still the way to go in the future, but for now it's much less practical due to its companion header not being truly widely supported yet and on its way to be replaced soon.

[cak]

Thanks for the suggestion, I'll get this added!