Added Datafold in partners page and uploaded logo.

For the contribution process and guidelines, please check Integrate with GitLab.

Checklist:

1. Application's data added to data/applications.yml (to the end of the category)
2. Application's logo added to source/images/applications/apps/ (optional, but recommended)
3. Mike LeBeau's review - cc/ @mlebeau
4. Tech writer's review