Change IMAP to try oauthbearer first.

Change IMAP to try oauthbearer first.

$imap_authenticators says if it is unset, the authenticators from
most-secure to secure will be tried.  It makes sense for oauthbearer
to come first, like with POP.

To make this change backwards compatible, it depends on the previous
commit, which changed imap_auth_oauth() to return IMAP_AUTH_UNAVAIL if
oauth is not configured or explictily requested.