Do not send Uncategorized groups to Publishing API

Story: https://www.pivotaltracker.com/story/show/82281760

As we are adding the latest feed, we no longer need to display the "Other" section at the bottom of a sub-topic.

This PR stops uncategorized groups, which power the "Other" section, being sent to the Publishing API. It also adds a note to the user on the frontend warning about this behaviour.