Let instance admins set the default visibility for blogs on their instance.
In particular, some instances will want to have all blogs default to "Public". But we might also imagine admins creating an all-Private instance, maybe serving as a backend for a note-taking app, etc.
- Create new default_visibility field in [app] config section
- It should accept a string (so it's human-editable)
- The string should map to the int value used in the collVisibility type when reading this value
- Enable changing from Admin dashboard
- Creating a new user / collection should check this configured value and set the value for the new collection
- Changing the config value shouldn't change existing blog values