Remove Link rel=next for authzs and new-certs. (#4303)

An early version of the spec indicated that clients should process issuance
sequentially, following Link rel=next from an account URL to an authz URL, to a
new-cert URL. However, the spec has long since moved to putting these URLs in
the directory.

Certbot nominally supports either; This change consolidates on always using the
directory, simplifying things and making the transition to the latest ACME spec
easier.

* Revert "Revert "Remove Link rel=next for authzs and new-certs." (#4277)"

This reverts commit 11ec1eb911.

* Save new_authzr_uri with account for older clients.

* Add test that new_authzr_uri exists in regr.

* Restore backwards compatibility for new_authzr_uri.

* Fix account_test.

* Add test for deprecated URI argument to request_challenges.

* Review feedback.

* Fix test

* Add omitempty to new_cert_uri.

certbot/interfaces.py

@@ -32,7 +32,7 @@ class AccountStorage(object):
         raise NotImplementedError()
 
     @abc.abstractmethod
-    def save(self, account):  # pragma: no cover
+    def save(self, account, client):  # pragma: no cover
         """Save account.
 
         :raises .AccountStorageError: if account could not be saved