Added list view and invoice view

Added list view for invoices as well as the general invoice view.

Invoice details are not yet created.
Added due date and status to invoice.php
Added "Print Invoice" button
Updated DB
Added start of billing/new.php
started new invoice file
added: create new invoice, add items to new invoice including auto-calculations, send notification on new invoice.
added ability for admin to mark invoice as paid and also to not show draft invoices to non-admins
updated DB for staging
integrated Stripe for payments
Minor updates to stripe form
updates to code layout in billing
Converted to use namespace and composer
Merge branch 'composer' into staging
added composer.lock to gitignore
Merge branch 'composer' into staging
updating composer deps
updates to composer
updated composer
Updated composer to psr-4