[Tails-dev] review and merge feature/7752-keyringer

Ticket: https://labs.riseup.net/code/issues/7752
Branch: feature/7752-keyringer into devel
Milestone: 1.3

The doc is ready now, so please have a look and merge.

I wonder whether we should spend energy on writing tests for that.
I feel like we have more urgent tests to write, to be honest.

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sajolida