Improve Django admin view for invoices and transferwise payments.

This PR adds the following features:

• Clickable Invoice PDF path.
• Clickable TransferWise Bulk Payment CSV path.
• Allow searching invoices by date.
• Clickable JIRA Timesheet path.
• Action to mark invoices as paid in bulk.
• Action to mark invoices as manually approved in bulk.
• Show invoice string (which includes its date) instead of UUID.

Testing Instructions:

Invoices

1. Make an invoice, and see it in the list and detail admin view to ensure the fields match what you expect given the requirements.
2. Get line items from JIRA for that invoice through the shell, as well as turn it into a PDF and upload it to GDrive.
3. Check the list and detail admin view again and ensure the fields match what you expect given the requirements.
4. Make sure all links work, including the PDF link for GDrive, and the JIRA timesheets link for the given provider.

TransferWise Bulk Payments

1. Make a TransferWise Bulk Payment, and see it in the list and detail admin view to ensure the fields match what you expect given the requirements.
2. Create individual payments for the TransferWise Bulk Payment through the shell, as well as it turn it into a CSV and upload it to GDrive.
3. Check the list and detail admin view again and ensure the fields match what you expect given the requirements.
4. Make sure the CSV link works when it's pointing to GDrive.