New content control setting: Add topics

Content control settings permit the user to make modifications to topic data, but there is no setting that controls whether a user can extend a map with their own topics.

Suggested new content control flag: "Add topics"

When this flag is off, users cannot add any topics to a map, and can only make the modifications to existing topics that are permitted by the other flags. This is the current implicit behaviour.

Switching this flag on would permit users to add their own topics to a map. Modifying existing topics would still be controlled by the other settings, and deletion of existing topics is still not permitted.

This would enable a map containing some protected base information to be elaborated upon by other users.