Consider putting CORS-accessible JS in the source view

Our source view currently never displays JS source code.

This is for two reasons:

* The sample-count-per-line information for JS JIT code is currently incomplete ([bug 1441689](https://bugzilla.mozilla.org/show_bug.cgi?id=1441689))
* Most JS code is not CORS-accessible, so we'll have to proxy the requests through the browser.

However, some JS files **are** CORS-accessible. At the moment we don't even attempt to fetch those files.

We could try to fetch the JS file always, and if it happens to succeed, display it even without accurate sample-per-line information. This would address a subset of use cases.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-749)