auth: use unsafe secret stash if secure is unavailable

[abhinav]

Headless Linux machines (Docker environments and the like) can't easily get access to GNOME Keyring or other secure storage. In such cases, we should fall back to plain text storage, but warn the user about it so they know what's going on.

Resolves #293

[abhinav]

This change is part of the following stack:

• 298 ◀

_{Change managed by git-spice.}