Update notices to success or danger flash msgs

Be clearer about a success or failure using Bootstrap alert styles.

:notice and :alert are still used by inherited_resources and responders
gems, customising the flash keys doesn’t work because of the old
versions being used.

* Explicitly set :success or :danger flash messages in app
* Coerce :notice to a success message
* Coerce :alert to a danger one
Merge pull request #302 from alphagov/conspicuous-affirmations

Update notices to success or danger flash messages