Add featured image functionality

## Description
For each item (project/organisation), they could benefit quite a bit with a featured image that would be used in the header of the item page or as a thumbnail in the card. This would then translate well for Open Graph.

## Screenshots

[WIP]:
We'd include mockups here. Please share them in comments and we'll update here with selected one.

## Files

N/A (for now)

## Tasks

- [ ] Create mockups of what this would look like.
- [ ] Add `Featured Image` column to Working gSheet.
- [ ] Write code to save image to s3.
- [ ] Write code to create version with overlay of AfricanCOMMONS logo for social sharing.
- [ ] Update project page to display featured image.