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).
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