Twenty Fourteen: Clean up post thumbnail namings

[iamtakashi]

Currently post thumbnail related stuff in the theme files has some inconsistencies such as image sizes named featured thumbnail-xxxand CSS classes like featured-thumbnail, has-featured-image, and so on. I'm going to rename all these with post-thumbnail-xxx for the sake of consistency.

Although I'm going to leave the screen reader text "No featured image" in twentyfourteen_post_thumbnail() as it is because that's a user facing text.

Also I'm going to remove the image size featured-thumbnail-formatted because the maximum width the thumbnail can be is 672px.

[lancewillett]

In 25971:

Twenty Fourteen: clean up post thumbnail naming. Props iamtakashi, fixes #25718.