Add Preview in Playground buttons to plugin install screens

[tellyworth]

Proposal: add Preview buttons to plugin install screens within wp-admin.

Background:

The WordPress.org Plugin Directory now supports a Live Preview feature based on Playground; see ​https://meta.trac.wordpress.org/ticket/7251 for context.

You can see an example working on the ​https://wordpress.org/plugins/convert-to-blocks/ plugin. Previews are currently only enabled on plugins where the developer has opted in and provided a blueprint.json file (which provides setup and configuration info).

The preview button on the web site works simply by redirecting from https://wordpress.org/plugins/PLUGIN-SLUG/?preview=1 to a playground.wordpress.net URL which loads the plugin's blueprint file. It opens the playground URL in a new window using target="_blank".

I'm proposing that we do essentially the same thing from within wp-admin, on screens with an Install button for plugins: add a Preview button, which opens https://wordpress.org/plugins/PLUGIN-SLUG/?preview=1 in a new window.

The wordpress.org plugins API already returns a preview_link property containing the appropriate URL for plugins with a working preview.

I'll attach a diff with a minimal implementation; this is intended as a starting point for testing and discussion, not a finished patch in its current state.

[tellyworth]

Initial draft patch for discussion and review

[tellyworth]

Here's what the button looks like in the plugin search/card view.

[tellyworth]

And here's what the button looks like on the plugin detail view (bottom left).

[tellyworth]

I added two screenshots showing the current state of the UI - it's basic and needs some feedback from design as to whether or not it's acceptable.