Upload can sometimes be very slow or fail

A user reported some difficulties when uploading a profile:
They managed to profile a busy Firefox session (opening Facebook in many container tabs), then capturing the profile locally.
But when clicking on Upload (menu) -> Upload (button), a first attempt didn't finish after 20+h(!) waiting, a second attempt almost finished but failed with "no connection" or similar.
They could download the profile JSON file to their computer. The file was about 30MB.
They sent it to me through Dropbox, which would show that the network was probably not at fault; and then I could upload it to profiler.firefox.com without problems.
They're in a big city NW of the USA, with an Xfinity connection. www.speedtest.net reports 114.6 Mbps down and 5.92 Mbps up.

I guess we can't do much with this report by itself, but here are some thoughts that could help our users and/or us to find and explain these issues:

* Could we give more information during the upload? E.g., actual number of bytes sent, stats on speed, etc. Warn when nothing happens for a while.
* Provide a way for the user to give up on an upload, and/or retry.
* In case of error, give as much information as possible.
* Maybe record logs in the server, and give some identifier in case the user wants to tell us about it.
* Offer alternate ways to upload, e.g., accept a URL (like a dropbox URL) from which our server could download the profile. At least it would be a workaround when the profiler.firefox.com really can't happen.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-356)