[Feature Request] Upload documents in admin area

[mammutserver]

Is your feature request related to a problem? Please describe.
I just installed FOSSBilling because I needed some form of customer portal. However I realized that you can only generate invoices via the admin panel. People that are alreasy using a different software to generate invoices can't upload them to FOSSBilling to give their customers the ability to view them online.

Describe the solution you'd like
Allow admins to upload PDF documents, tag them (invoice, offer etc.) and link them to a customer. The customer should receive a mail that a new document is available in their account.
For invoices it would be useful to be able to add the total amount of the invoice and the invoice number so that uploaded invoices can be processed through payment gateways in the system.

Describe alternatives you've considered

Additional context