[electrum] allow disabling SSL when using localhost electrum server

Summary: This makes testing the server easier, without having to patch the Electrum ABC code everytime. Just open the network dialog, type 127.0.0.1 in the host box, and then you can uncheck "Use SSL".

Test Plan:

cmake -GNinja -DBUILD_CHRONIK
ninja
src/bitcoind -chronik -chronikelectrumbind=127.0.0.1:50001:t

Start Electrum ABC, connect to 127.0.0.1:50001 without SSL

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18554