Use new ENABLE_PUBLISHER feature flag

ENABLE_MKTG_SITE has been serving double duty to both indicate that
an Open edX installation is using a Drupal marketing site AND
is using the Publisher workflow tool for managing course metadata.

But now that publisher-frontend is available and the Publisher
feature is no longer tied to the marketing site, we want to tease
apart those two concerns. Hence ENABLE_PUBLISHER.

Only really used so far in Studio, to control which fields are
shown (if using Publisher, some fields are only editable in
Publisher).