Media block should use responsive image sizes based on it's container

[busbyk]

Media blocks in richText fields use viewport width based responsive sizing which doesn't work when embedded in a smaller container. Plus we should have an additional, smaller size.

https://www.loom.com/share/d49426c1009d44879abce2f3f55c447b

Applies to inline media blocks as well.

[rchlfryn]

@busbyk can you confirm this done in #867?

[busbyk]

@busbyk can you confirm this done in #867?

No, it's not. The MediaBlock still uses viewport based max width classNames to handle it's responsive sizing. We need to update those to be based on it's parent element instead of the viewport because they don't work as expected when the MediaBlock is embedded in a smaller container - the posts layout for example.