Add image thumbnail to media list in microsite dashboard

[finnlewis]

By default, installing LocalGov Microsites 4.x, creating a coupe of microsites, and visiting the group/2/media tab, we see the media that is part of site 2:

It might be nicer for content managers to see the thumbnail of the images or other media :

[finnlewis]

@ekes / @stephen-cox is this best done with a view config in config/overrides like:

https://github.com/localgovdrupal/localgov_microsites_group/blob/4.x/config/overrides/views.view.group_members.yml

Then add something similar to :

https://github.com/localgovdrupal/localgov_microsites_group/blob/4.x/localgov_microsites_group.module#L62-L67