Save the server list in Manager as fallback in case cloud provider is inaccessible about outline-client

[fortuna]

I believe the access token we get from OAuth is expiring.
It used to be the case that the token would last several months. But since then DigitalOcean changed their system to make the tokens expire sooner.

We probably need to use the Refresh Token Flow as explained here: https://docs.digitalocean.com/reference/api/oauth-api/

[fortuna]

I guess it would also be helpful to save the server list so we can manage them even if DO is inaccessible. You wouldn't be able to delete the server, but at least you can do the rest.