MF2 image support

I've had a a to-do to add meta box to the media attachment screen to allow a user to choose whether an image is photo post (`u-photo`) or featured (`u-featured`).

I finally found `attachment_fields_to_edit` so I can move forward with that now.

Question is, should it be in the Microformats 2 plugin, or the IndieWeb plugin? I was thinking IW since not everyone necessarily will have mf2 installed, or the need to if using a mf2 ready theme.

@dshanske made the point that the plugin is designed to play well with themes if they declare support for mf2.

I can see logic for keeping all things mf2 in the MicroFormats 2 plugin, but I'd also like to see as many people get to use the photo option. It's easy to get confused as to how to include an image when POSSEing to Twitter or Facebook.

Do you have any thoughts @pfefferle ?

Start by reviewing the existing media attachment handling around the attachment_fields_to_edit hook and compare the responsibilities of the Microformats 2 and IndieWeb plugins. The issue is ready only after the plugin ownership is decided and the expected photo-post and featured-image behavior is specified.

Written by the indexing model from the issue text.