Add A/V Thumbnail support to our Default Themes

[kirasong]

Per @wonderboymusic's ​recommendation, we should either enable A/V thumbnail support globally, or specifically within each of our default themes shipped.

Audio and Video playlists are best viewed including their covers, and it feels broken to not do so along with the other support added in 3.9.

Related: #26631

[lancewillett]

Possibly related: #27243 — should this ticket be in "Bundled Theme" component or "Media" maybe?

[kirasong]

Yes, it should, thanks! Done.

[wonderboymusic]

27460.diff​ turns on thumbnail support for attachment:audio and attachment:video. The thumbnail meta box was already showing up if theme OR post type support was turned on, so I changed the other conditions to check the same. I wrote those original conditionals in 3.6, when there wasn't a pressing need for images, there is now since playlists.

Rather than having every single theme on Earth turn on the feature, we're turning it on at post type level, so the theme only needs to turn it on if a plugin for some reason turns it off.

[wonderboymusic]

In 27657:

Turn on thumbnail support for attachment:audio and attachment:video. Change conditionals to check for theme OR post type support when determining whether to turn on the Featured Image UI pieces in the admin.

Fixes #27460.

[wonderboymusic]

In 27659:

When inserting a video shortcode into the editor, set the poster attribute automatically if the video has a featured image attached to it.

See #27460.