Explore automatically expiring uploaded profiles after 6 months

Moving a prior discussion into this issue.

Uploaded profiles should expire automatically after 6 months and get deleted.

**Why?**

1. **Data hygiene**. [Lean data](https://www.mozilla.org/en-US/about/policy/lean-data/) practice:
> "try to keep [data] for only as long as necessary".
1. **Half-life of profile relevance.** Hypothesis: After 6 (or 12?) months _(or 4/8+ Firefox releases)_ a profile of Firefox is unlikely to reflect the state of Firefox and/or the profiled website.

Cost savings on storage is a non-issue.

**Why not?**

1. Maybe one day there is a rare case that requires looking at an old profile?
1. Reference profiles uploaded for training purposes should be kept (can also be solved by uploading and loading from URL)

**Possible investigations?**

* Collect telemetry about the age of opened profiles
* Share opt-in option to limit expiration of a profile before uploading (pastebin style)
* Share opt-in option to extend the expiration of an uploaded profile
* Change expiration heuristic to time after last opening, so actively used profiles stay online.

Thoughts?

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-56)