devel branch FTBFS: Electrum was removed from stretch-backports

If we can install the version from sid, let’s do it. Otherwise, let’s import into our custom APT repo the latest version that we can install.

Feature Branch: https://salsa.debian.org/tails-team/tails/merge_requests/11

Related issues

• Blocks #16209

Original created by @intrigeri on 16642 (Redmine)