Absolute URLs for links appearing in feed

note that this changes the oft-used byline()
Merge branch 'master' into issue2954_relative_urls_for_atom_feeds

Conflicts:
	app/helpers/series_helper.rb
Use ts(...) for series_list_for_feeds.
Merge pull request #736 from openendings/issue2954_relative_urls_for_atom_feeds

Absolute URLs for links appearing in feed