Use kilnauth or equivalent technique for authentication

[benkomalo]

Corresponds to a TODO in the code, but deserves an explicit issue, IMO.

There is an existing kilnauth extension to avoid having to store your credentials in plaintext. Under the hood, it uses a simple cookie jar. We should do something similar. https://developers.kilnhg.com/Code/Kiln/Group/Kiln-Extensions/File/kilnauth.py

[sophiebits]

I verified that the cookie that kilnauth stores works properly with the API.

[kamens]

FWIW I use this extension and it works well.

[csilvers]

Done.

That said, it could be better integrated (to prompt for a username/password if one is missing) -- but I don't know how to do that.