Add ability to set custom picture for entities

[thybag]

Hi @nathan-gs, Hope these look okay, two minor quality of life additions.

• Add picture property to entities. Setting this with a URL will override the default photo for markers.
• ~Add map_height (int) to card. This sets map min-height and can be used to override/force a preferred height for the map itself.~

[nathan-gs]

Do we really need a map_height vs card_size option?

[thybag]

Hi @nathan-gs, I'd missed the card_size when i first added. I agree its pretty much a duplication so have removed from the PR 👍