Rename pair command, fix systemd unit, do not send notification port

This update requires version 1.3.3 of the app for full functionality.

• Rename pair command from "pairing" to "pair"
• Run the correct command in the systemd unit
• Do not send notification port
• Other minor changes

Drop support for user installation, new command line interface and version code

• Drop support for a user installation under $HOME to achieve a simpler and safer Makefile which does not install any dependencies
• New, simpler command line interface, with an extra command to start pairing
• Version command

Run commands in shell

• Run commands in shell to allow more complex ones

Notifications toggling and configuration error checking

• Toggle notifications using SIGUSR1 signal (e.g. killall -SIGUSR1 a2ln)
• Check whether all configuration files exist like keys
• Remove setproctitle as a dependency as it is unnecessary
• Do not run commands in shell to improve security

Fix systemd service when installed as user

• Fix the systemd service path to the binary when it is installed as a user

Stop server when notification or pairing server died

• Stop the server when either the notification or the pairing server died

--body-format option and a way to not start the servers

• --body-format option to specify a custom body format
• A way to not start the pairing or notification server (so there are no unnecessary ports open)

Fix build

• Fix Makefile

Pip packaging

• Change project structure to allow future Pip support
• Installation using pip

Support for Python 3.8

• Support for Python 3.8