Twenty Nineteen: Add starter content based on the theme demo

[allancole]

Originally reported by @joyously in the Twenty Nineteen GitHub repo.

As @celloexpressions states:

Starter content is a relatively new feature in core, dating to WordPress 4.7. As a relatively new feature, and given that bundled themes should be "state of the art," it seems like a mandatory element of Twenty Nineteen. We should also consider the goal of starter content to improve the new user experience with fresh installations of WordPress. Leaving this out of Twenty Nineteen would be a regression relative to the current fresh-install experience with Twenty Seventeen.

Let’s go ahead and add some starter content so users have an example of how to recreate the content in the demo.

Included is a patch that replicates the demo content and adds support for it. There are two additional image files attached to this ticket that need to be added to the /img directory of twentynineteen.

Original ticket here: ​https://github.com/WordPress/twentynineteen/issues/216
Original pull request here: ​https://github.com/WordPress/twentynineteen/pull/510

[allancole]

This patch adds theme support for starter content and includes the starter content itself. There are two image files that are also attached and need to be added to the twentynineteen/img/ directory for the starter content to work properly.

[allancole]

Portrait placeholder image. Add this file to the twentynineteen/img/ directory.

[allancole]

Landscape placeholder image. Add this file to the twentynineteen/img/ directory.