refactor: migrate FRONTEND_APP_PUBLISHER_URL off FEATURES-as-dict

The URL was only read via settings.FEATURES.get with an inline False
default and was never defined in a common envs file (only in
cms/envs/devstack.py). Add an annotated flat setting to cms/envs/common.py,
defaulting to None (the idiomatic 'no URL' value, consistent with the
sibling keys in the course settings context) and matching the prior
falsy default, then read it directly at the CMS-only caller.