Password saving (or at least username)

[daboross]

Currently, screeps-rs requires entering username/password on every startup. During testing, at least, this can be very tiresome.

Ideally we could just store an API token in a local file, but they seem to expire fairly rapidly. Maybe a good solution would be to store username + API token + password if "save password" box is checked?

I'm not entirely sure how to do this securely on each platform, though, so saving password should probably wait until we figure that out.

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Note on database: We currently use rocksdb to store terrain caches. IMO we should store servers/usernames/passwords in a separate database or file, managed separately.