Allows notes to be required project-by-project

Some clients require custom notes each week, for those clients, we can
flag an extra notes field which will be automatically included in each
invoice.

To turn it on for a given project toggle `requires_notes`

`Project.find(1).update(:requires_notes => true)`
Require project summary notes when marking ready.

If a user has not added project summary notes to a
project which requires it, then validation will fail
when they attempt to mark the invoice-week timesheet as
ready to invoice.

Relates to #18