from-url with http://localhost URLs does not work in Safari

Browsers generally do not allow https pages to `fetch` from http pages ("mixed content"). However, Firefox and Chrome consider `http://127.0.0.1` potentially trustworthy and allow https pages to communicate with it. But Safari does not. This means that, only in Safari, `https://profiler.firefox.com/` is prevented from importing a profile from `http://127.0.0.1:8000/`, for example.

This completely breaks the "import from local server" use case. I'm not sure what to do about it.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-418)