Don't let the browser show a cached transition page

In user research users have often been confused by the inconsistent state of mappings, hits and flash messages when using the back button. This PR is to address ticket #68870422.

This may be a contentious change, which is one of the reasons I've opened the PR for some discussion about the problem.

* Avoid showing a user an inaccurate mappings list when a user hits back
* Avoid showing flash messages multiple times when a user hits back
* Avoid showing incorrect summary tables on hits pages after editing mappings