[Invoices] Improve UI for managing sponsors

### Describe the problem

Currently there's a way to edit sponsors but no way to get to this page.

### Describe the feature/solution (optional)

Give users the ability to navigate to these pages and improve UI for managing sponsors

Start by locating the existing sponsor-editing page and the current invoices navigation. Add a discoverable path to the sponsor-management pages and refine the UI shown in the issue screenshots; done means users can navigate to the pages and manage sponsors through the improved interface.

Written by the indexing model from the issue text.