This is a fairly substantial change, which adds a new configuration file
to the user's `$RAD_HOME` that includes node configuration, including
the alias, and also makes node aliases required.
When running `rad auth`, the user is now prompted for an alias, which
defaults to `$USER`.
When running `rad self`, the alias is now shown.
If the user runs radicle without a config file, the defaults are loaded,
and `$USER` is used as the alias.
We consolidate the setup done by `rad checkout`, `rad clone` and `rad
remote add`, so that they all check for node aliases, and are all
capable of setting up tracking branches etc.
Alexis Sellier