Replace the import-time nav set-up with a settings-based one.  This is
more flexible ultimately although it doesn't allow extensions to easily
add their node automatically.  However this is probably a good thing
overall as it isn't easy to remove nodes in the previous impl.

Fixes 426

Features:
* Basic crud
* Stats view

Pushed the form-driven functionality back onto its own page within order
management.  The dashboard index is now static and the page is more
coherent as a whole.

I moved the ``OderSummaryView`` from the order section in the
dashboard to the main dashboard index and merged it with the
``IndexView``. I removed the summary template and merged the
relevant data into the dashboard template.

This can still be extended easily, but is now prettier for single option
downloads which is the default.

Use the fns in app.dashboard.nav to edit