Use type instead of http_status in Mapping model

This is a first step towards this goal, and will have broken lots
of things. The mapping model specs pass right now, though...
Make bulk tagger view model specs pass again
Make mappings batch model specs pass again

Now all model specs are passing.
Make mappings_from_host_paths specs pass again
Make Whitehall mappings import specs pass again
Make MappingsBatchOutcomePresenter specs pass again
Make mapping controller specs pass again
Make helper specs pass again

There are some quite horrible things here, but at the moment I'm just
trying to get all the specs passing again. When I get onto the features
I'll have more of an idea about which helpers we actually need, and
will revisit this to make the helpers better.