Issue #56, remove diagnostics tools menu, remove missing document list functionality
Issue #56, remove fabric file installation detection
Don't import tags code from linking app, violates separation of concerns of child apps
Remove tempory_navigation_links to a more meaningful extra_navigation_links
Simplify and optimize smart linking code and views, remove now unused LINKING_SHOW_EMPTY_SMART_LINKS configuration setting (issue #56)
Namedtuples have an __iter__ attribute but are not iterable in the navigation list sense so test against list subclasses instead