Fix regression related to #556

Newly created, unsaved stories had their section list rendered incorrectly
since the width was based on the spacer width.  When a story is unsaved,
no spacers are shown.  This sets a default spacer width so the calcualtion
works whether or not the story is saved.
Merge branch 'hotfix-556-section_list_on_two_rows' into develop

Conflicts:
	README.rst