add placeholder image constant; add to partner detail

[saragibby]

Description

Use the member placeholder image on the partner detail page for instances when the partner member does not yet have a profile image uploaded.

Also, moves the person placeholder image to constants.

Fixes #581

Screenshots (if appropriate):

Example partner that has a member without a profile image:

Types of changes

• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Running app locally checking "Do It Best Corp" who currently has member without a profile image.

Checklist:

• [ ] I have updated the documentation if necessary
• [ ] I have updated the stories as necessary
• [ ] I have updated the specs necessary

[vercel[bot]]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/that-conference/that-website/gfeut9d7e
✅ Preview: https://that-website-git-sara-581partner-detailmissing-member-image.that-conference.vercel.app

[saragibby]

Sure thing. Cool to see all the things getting used and these edge cases surface for us to get fixed.