Remove can_have_summary? - all Edition types can have one now
All editions now require a summary
When importing, generate a summary if it's missing from the body

We use the new Parsers::SummariseBody to do so.  Or we will later once it has an implementation (right now it just returns the body).
Fill out Parsers::SumariseBody

Turn the body into html using Govspeak, remove the html using ActionView::Base.full_sanitizer.  Then we try to truncate to 140 chars, allowing for the 140th char to be inside a word and doing the truncation at the end.  We've rolled our own because this is the opposite behaviour to rails own truncation.  It is conservative to make sure you don't go over the limit at all whereas our limit is soft so going over it is ok.
Fix cuke helpers to always provide summary
Add a data migration to summaries the body for editions with no summary