Only prompt for project notes once-per-invoice.

Seemed silly prompting for this both weeks and also validating across
two weeks when all it would do is create busy-work for end users and for
the invoicer to make sure the two sets of notes format nicely together.

cc/@toddkaufman -- just making a judgment call on UX here based on my 
own testing, let me know if I'm off base

Relates to #18