Invoice CRUD moved to separate routes

Removed invoice modal in favor of full page based invoice CRUD set up.
Modals are not always the best choice for mobile devices. Made sure to
include project info on the invoice new/edit form for user reference.
Added invoice#show template

Although not usually directly addressable, added invoice#show template
for completeness.
Moved payments to separate CRUD pages

Like invoices, the payment modal isn't really great on mobile devices.
So the payments are now separated to CRUD pages.
Improved payment/invoice CRUD