rbw config: null is not substituted with proper (sub)domain when using bitwarden.eu

[merv4]

This is an extension of #148.

The domain (bitwarden.eu) has identical plumbing to its counterpart (bitwarden.com), but it is not handled the same by rbw. This leads to authentication failure when using most rbw subcommands. There also is no error message, making it difficult to diagnose.

When null is present, and base_url + ui_url are set to bitwarden.eu, null should autofill bitwarden.eu instead of bitwarden.com .

Here is the current behavior:

base_url = api.bitwarden.eu
identity_url = null = identity.bitwarden.com (!)
ui_url = vault.bitwarden.eu
notifications_url = null = notifications.bitwarden.com (!)

Here is the desired behavior:

base_url = api.bitwarden.eu
identity_url = null = identity.bitwarden.eu
ui_url = vault.bitwarden.eu
notifications_url = null = notifications.bitwarden.eu

It would be even nicer if setting base_url to bitwarden.eu causes ui_url to be vault.bitwarden.eu when null, but we can call it a stretch goal.