Electrum offline signing

The used Electrum version 2.7.9 has an offline signing bug - tries to connect to the server and times out failing to sign the transaction. This bug makes Tails unusable as a secure bitcoin wallet. This bug is fixed in the >=2.7.11 versions of Electrum.

Related issues

Related to #12545 (closed)
Blocked by #15022 (closed)

Original created by @zivke on 12323 (Redmine)

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