Feature/handle manual invoices

Added admin system for admin for manual payments (invoices) registered by users. This feature makes it much easier to handle such payments. 

Generally *manual payments* goes through of three phases:

1. **Uncompleted** - Newly registered manual payment request. These needs our attention as we need to write an invoice and send it to the customer
2. **Invoiced** - We have created an invoice and sent it to the customer. We are waiting for it to be paid by the customer
3. **Finished**  - The customer has paid the invoice and everyone is happy!