Allow optional pages by adding page_desc.is_optional_page attribute.

course/flow.py

course/models.py

course/page/base.py

course/page/choice.py

course/page/code.py

course/page/inline.py

course/page/text.py

course/page/upload.py

course/templates/course/flow-completion-grade.html

course/templates/course/flow-confirm-completion.html

course/templates/course/flow-page.html

course/views.py

doc/flow.rst