Use effectiveAvatarEmail() for avatar lookup

Support the new avatarEmail account field which allows users to
select which email address is used for avatar lookup via the
${email} template variable, independent of their preferred email.

This change uses Account.effectiveAvatarEmail() which returns
avatarEmail if set, otherwise falls back to preferredEmail.

Depends-On: I045c075b1bbd7ee7eabb1a144d9b577e907daf5f
Change-Id: Id4bbc0e17e15b2b558ed736aab99e41a20855931