Integrate OAuth login/logout to CUPS Web Interface

[michaelrsweet]

Need to have a way to start the OAuth authorization flow and receive the redirection to authorize via OAuth.

Probably use a cookie to pass the authorization credentials from a browser context, which can be cleared when needed to logout.

[michaelrsweet]

Getting there, still need to do some infrastructure stuff:

• cupsOAuthCopy (and oauth_load_value) need to look in /etc/cups/oauth if the per-user stuff doesn't have anything, then we can have a temporary home directory vs. pointing at /etc/cups.
• Need to better address OpenID implementations that don't use JWTs for bearer tokens, and figure out why Entrada ID has a bogus signature.

[michaelrsweet]

OK, shared info between user and system OAuth directories is now implemented, and cupsd now uses the userinfo endpoint to determine whether the bearer token is valid (and to get the user info, obviously).