Added representatives and the ability to import clients

[TreyWW]

Checklist

• Ran the Black Formatter on any new code (checks will fail without)
• Made any changes or additions to the documentation where required
• Changes generate no new warnings/errors
• New and existing unit tests pass locally with my changes

What type of PR is this?

• ✨ Feature

Description

Added the ability to import existing clients to invoices. This is a great feature and makes it much faster to create invoices.