Don't redisplay mappings progress messages

Disable caching on responses which include feedback on progress to avoid confusing users who hit the back button.

We don't want to disable it on every page because:
* this would unnecessarily slow down the user experience
* values in forms would no longer be retained when a user hits the back button

Kudos to @fofr for the meat in this Pull Request pie. :meat_on_bone: :cake: