Enable Twitter unfurl image configuration in Django admin

**User Story:**
As a Grants Ops user
I want to be able to update the in Django
So that I can keep our marketing materials up-to-date

**Acceptance Criteria**
GIVEN that I am in Django
WHEN I click to change the carousel image
THEN I can upload a

**Product & Design Links:**

**Tech Details:**

**Open Questions:**

**Notes/Assumptions:**

Start in the Django admin entry point for the carousel image and determine how the Twitter unfurl image is currently represented. Confirm the intended field and upload behavior with the issue author; done means a Grants Ops user can upload or change that image in Django admin and use it for Twitter unfurls.

Written by the indexing model from the issue text.