add submitted-at time

see #11
Refactor invoices so to save sent_to_harvest_at
footer margin
improve naming
Add a todo route for invoices harvest needs

Required lots of changes:

* Simplified search for timesheets when creating invoices
* A system config to keep track of an 'epoch' or reference point for
invoices to pivot off of so we can ask whether any given week is invoice
week
* A couple interesting queries hunting through for either timesheets-that
demand-an-invoice or invoices that have since seen timesheets or entries
be updated and fallen out of sync.
Implement /todo route

closes #11