[2.x] Simplified featured image schema for blog posts

[caendesilva]

Okay, I'm gonna be honest here. I in theory love that HydePHP can assemble dynamic featured image captions. But in practise, it doesn't work. I wrote the code and designed the schema, and I still can never remember what the accepted properties are. I think for v2, it's worth the BC break to simplify these. Stuff like altText and licenseUrl is just too hard to remember.

Instead, this would be much better, to have a literal caption field instead of dynamic data items:

image:
    source: how-to-turn-your-github-readme-into-a-static-website-cropped.png
    alt: Example of a static website created from a GitHub Readme
    caption: Static website from GitHub Readme

If alt text is not set, but caption is, then we can use the caption as the alt.

If we wanted to have fun, we can have a migration script that converts the old files into the new format by assembling the fields according to the v1 rules.

[caendesilva]

We may consider backporting support for the new syntax to 1.x.