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
Merge pull request #313 from alphagov/dont_cache_pages_with_mapping_progress_message

Don't redisplay mappings progress messages