Figure out what to do with optional Electrum dependencies on Buster

Originally created by @intrigeri on #15189 (Redmine)

See cb392c17. Let’s just wait a bit, by the time we’re closer to Tails 4.0 things should be back in shape, hopefully, if someone cares about it enough in Debian :)

Related issues

Related to #15390 (closed)
Related to #15452 (closed)

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