473 question tags

Ready for review. The part I'm most unsure about is the way `@ability` is created and used in the tests, and its interaction with user and mission. But all tests are passing! 
I think I was slightly wrong about this. You should be able to delete a standard-copy tag /only/ if it is not attached to any standard-copy objects in the mission. Otherwise if you import a tagged standard question and then decide you don't want it, you're stuck with the tag forever. I've updated the integrity document to reflect this. Sorry!