Plugin enable/disable now checks marketplace 'admission' status and can also disable a shadowed synced copy

Enabling a plugin now checks whether its marketplace has been "admitted," and if so, cleans up any stale local/user overrides that had disabled it under both its git and non-git marketplace ids. This prevents a newly-admitted synced plugin from being silently shadowed by a leftover disable override. A related synced-plugin-shadowed warning suppresses the managed-policy-lock message in this specific case.