In preview mode for long forms, allow clicking to expand

On previews for posts that are long enough to have the 'show more' button,

• regular tweets

• long form with formatting

This is supported in the previews feature.

But for long forms without formatting, it just shows the entire post, so it's not easy to see where the cut off will be in folks' timelines when they see the post for the first time, and where the media will appear.