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).

cms/envs/common.py

@@ -220,6 +220,9 @@ FEATURES = {
     # in sync with the one in lms/envs/common.py
     'ENABLE_EDXNOTES': False,
 
+    # Toggle to enable coordination with the Publisher tool (keep in sync with lms/envs/common.py)
+    'ENABLE_PUBLISHER': False,
+
     # Show a new field in "Advanced settings" that can store custom data about a
     # course and that can be read from themes
     'ENABLE_OTHER_COURSE_SETTINGS': False,