[electrum] initial monorepo integration

Summary:
This is a copy of the ElectrumABC repository as of commit https://github.com/Bitcoin-ABC/ElectrumABC/commit/9b72b3efe62a12f8f5abf883656af17992dd6c85

Two files related to CI and linting have been removed: .pre-commit-config.yaml and .cirrus.yml.
A new configuration file .isort.cfg has been added to mark the main packages as know first party packages to prevent isort from sorting them as third-party packages.

Permanent linter settings:

• The main modules are excluded from lint-python-shebang as a shebang does not make sense for non-scripts.
• Auto-generated files (protobuf and icons.py) are excluded from linters.

Temporary linter exclusions (to be fixed in separate diffs):

• flake8, because of remaining warnings from flake8 plugins (A001, A002, C401...)
• lint-python-encoding
• shellcheck
• flynt (low priority)
• mypy (low priority and difficult to fix)

Linter related changes:

• corrected spelling
• corrected and added bash shebangs
• added export LC_ALL=C.UTF-8 to shell

Other changes:

• adapted the build scripts to work when the directory mapped to ELECTRUM_ROOT inside the docker is not the root of a git repo

Test Plan:
arc lint --everything

Make sure Electrum ABC works: cd electrum && ./electrum-abc

Check the Help > Report Bug menu now points to the Bitcoin-ABC repo

Make sure the build toolchains work:

cd electrum
contrib/build-wine/build.sh
contrib/build-linux/appimage/build.sh
contrib/build-linux/srcdist-docker/build.sh

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14088