Change the content plans permissions so that everyone can create and edit plans

https://www.pivotaltracker.com/story/show/68637746
Only allow plans to be destroyed when they are empty

https://www.pivotaltracker.com/story/show/68637746
Add a better unauthorised message

At the moment it's a rather cryptic pundit error.

Also hides the delete button if there are plans associated.
Travis configs