tracking: upgrade u-featured property to "Draft" status

[sknebel]

u-featured has progressed and is now implemented in various places. Collecting examples

• it's published by at least one Wordpress theme: SemPress
• bridgy publish accepts and syndicates it: Bridgy Publish to Twitter should consider u-featured fallback for u-photo for article use-case snarfed/bridgy#741
• Readers: Together
• it is supported in XRay
• https://indieweb.org/featured lists some examples from the community
  ** TODO: indiemap.org could be used to get a larger overview over pages publishing it in the wild

Issues/conflicts:

• there was an issue with featured (and other) elements potentially also being implied to be photo properties, this was resolved through a change in the parsing spec: reduce instances where photo is implied microformats2-parsing#21

[sknebel]

issue: still discussion around it's use outside of the article use case, e.g. here: https://chat.indieweb.org/dev/2019-01-16#t1547671766812100

[dshanske]

Again, we have a definition issue here.

u-featured - a representative photo or image for the entry, e.g. primary photo for an article or subject-cropped photo, suitable for use in a link-preview.

Does u-featured have to be a photo or image? What about a representative video?