Improve social share preview for collections / subjects

Related to:
* #833
* #6054

e.g.
https://openlibrary.org/people/mekBot/lists/OL103955L/Childhood_Favorites/preview.png

### Describe the problem that you'd like solved

We don't have a great social preview for collections. It doesn't show a little text about the collection or image.

[Here](https://developers.facebook.com/tools/debug/?q=https%3A%2F%2Fopenlibrary.org%2Fcollections%2Fkingdom-of-hawai%27i-people) is FB's tool to improve social share.

Example link: https://openlibrary.org/collections/kingdom-of-hawai'i-people
What it looks like on Facebook:

What it looks like on Slack:

I'm sure it is just as bad on other apps.

### Proposal & Constraints

Perhaps we can grab the first few hundred characters and set it as the description.
Grabbing the first image would be nice too.

### Additional context

### Stakeholders