mutt should ship with a client_id and client_secret

Following up on #383 (closed), mutt_oauth2.py should work without ANY edits.

It currently leaves absent client_id and client_secret. These are magic numbers are "developer keys" that must be obtained from each e-mail provider. Without them the code is useless.

Mutt should ship with values there. This requires someone register at those sites to obtain the magic numbers. I assume one would want a mutt core developer to do that, following the procedure in mutt_oauth2.py.README.

(For what it's worth, Thunderbird has such keys and they are just checked in to the source code, in comm/mailnews/base/src/OAuth2Provider.jsm, so presumably Mutt can do the same thing.)