According to @bep, it is easier to undraft content by
editing manually the frontmatter of said content by
setting the draft flag to `false`, or removing it completely,
than to rely on the undraft command which is a source of
many bugs.
Fixes#4353
Move SummaryLength into the ContentSpec struct and refactor the
relevant summary functions to be methods of ContentSpec. The new
summaryLength struct member is configurable by the summaryLength config
value, and the default remains 70. Also updates hugolib/page to use the
refactored methods.
Resolves#3734
Bjørn Erik Pedersen