so i'm having some trouble getting this to do quite the right thing, even in the scenario wherein we don't try to recover  the story template's placeholder title/suggested title on load. summary of behavior:

- when creating a new story from a template, the first section title gets its template-defined placeholder
- when creating a new section, that section gets a generic 'Section Title' placeholder
- when loading a saved story in which a section lacks a title, that section isn't getting *any* placeholder at the moment.

that last could be handled by the handlebars template, but perhaps we'd prefer it to live in the view?

also i updated the section list view: when a section has no title it displays 'Untitled Section' rather than 'New Section' ... seemed a little more appropriate. however, this is also handled in the handlebars template rather than in the view code.
Create "default" handlebars helper. Move logic for providing generic section title placeholder attribute in builder to handlebars template. (#616)
Integrate localization gettext into "default" handlebars helper. (#616)
Merge branch '616-builder-placeholder-inconsistency' of https://github.com/patternleaf/atlas into patternleaf-616-builder-placeholder-inconsistency