Keep original Gecko profile and provide download access

This would help with investigating Profiler back&front-end bugs, by providing access to the raw data as captured by Firefox before any front-end processing.

Since it's intended for Profiler developers, it doesn't need to have a nice UI. E.g., the download could be initiated by appending something like "?download=gecko" to the front-end URL; Or with a js function call in the shell?

If there is a risk that this could expose private data that would normally be processed out, we would need to restrict this functionality to a few people, and/or encrypt the raw data.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-379)