Use match_origin_as_fallback when it lands

[kkapsner]

To get rid of the data-URL workaround we can use match_origin_as_fallback when it lands.

[kkapsner]

This is just a reminder for myself as I cannot include it yet: it produces errors (and warnings).

It has to be used in manifest.json (new property in content_scripts) and in main.js (call to browser.contentScripts.register).