Display upload progress while importing profile from Firefox/"add-on"

When capturing a profile from Firefox, the profiler.firefox.com page shows a bit of animation and "Importing the profile directly from Firefox...".
If that takes more than 30s, another message says "We were unable to connect to the Gecko profiler add-on within thirty seconds. [...] Still waiting..."
(Side-issue: we should remove "add-on".)

It would be great to have more information during this process, to inform the user whether nothing at all is happening, or the upload is indeed very slow.
A user suffering from Firefox using 100% CPU said they waited for more than ~5 minutes before giving up.

So I think we should at least display a count of bytes uploaded, to show whether data is coming through.

Other improvements:

* Show the current data rate.
* If the file size is known, _also_ show a percentage.
* Show the estimated remaining time.
* Coordinate more with the Firefox profiler (this would require Firefox backend changes as well), to better know what's happening _before_ the data starts streaming. E.g.: Request received, gathering main process data, gathered M/N subprocess profiles so far, zipping, and then uploading (with file size sent before the actual file).
* If "gathered M/N subprocess profiles" doesn't seem to reach M==N after a short while, offer the option to upload the profile as-is. ([bug 1648324](https://bugzilla.mozilla.org/show_bug.cgi?id=1648324) will attempt to add an automatic timeout on the backend side.)

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-282)