devilry_qualifiesforexam: Autoredirect to selectplugin when no status.
devilry_subjectadmin...PeriodOverview: Qualifies for final exam link.
devilry_header...Breadcrumbs: Document default_breadcrumbs.
devilry_subjectadmin: Started on a custom breadcrumb.

This is just a skeleton that will be extended for #307.
devilry_qualifiesforexam: Breadcrumbs.
devilry_qualifiesforexam: Handle breadcrumb urls in subjectadmin correctly.
devilry_subjectadmin: Use apptemplate from devilry_theme.
devilry_student: Use apptemplate from devilry_theme.

Fixes #340.