Change "format"s from dashed to underscored

Rummager doesn't seem to accept dashed document type values. It would be nice
to have a consistent value for the document type in Rummager and the format
field in both Rummager and Publishing API/Content Store.

I don't believe anything depends on the value of format and there are no live
HMRC manuals yet so there isn't a migration issue.

Non-HMRC manuals, which are managed by Specialist Publisher, currently have
dashed format names and there is at least one live manual. However, they are
not in GOV.UK search at the moment. I suspect that there may be trouble adding
them because they contain dashes.
Merge pull request #68 from alphagov/change_formats_to_underscores

Change "format"s from dashed to underscored