Onboarding: Set Newspack-specific sidebar widgets

[laurelfulford]

Is your feature request related to a problem? Please describe.
Right now when you set up a fresh Newspack site, you end up with a handful of sidebar widgets automatically set -- these are coming from WordPress, not anything in the Newspack plugin:

While some of these could be helpful, we never leave all of them; when this set of widgets is on a populated site (and not an empty one like in my screenshot), the sidebar can also get super-long.

Describe the solution you'd like
It would be super-handy if the Wizard could set a shorter list of sidebar widgets -- something like:

• A search widget (but with the appearance set so there's a placeholder instead of a label, and the button inside of the search box, using the icon instead of text -- this makes it look like the search box generated by the theme).

The HTML for the above block looks like this:

<!-- wp:search {"label":"","placeholder":"Search...","buttonPosition":"button-inside","buttonUseIcon":true} /-->

• A Homepage Posts widget, set to show 4-5 posts with settings appropriate for the smaller space:

The HTML for the above block looks like this:

<!-- wp:widget-group {"title":"Latest News"} -->
<h2 class="widget-title">Latest News</h2><div class="wp-widget-group__inner-blocks"><!-- wp:newspack-blocks/homepage-articles {"showExcerpt":false,"showAuthor":false,"postsToShow":4,"mediaPosition":"left","typeScale":3,"imageScale":1} /--></div>
<!-- /wp:widget-group -->

Describe alternatives you've considered
We've just been manually setting the above as part of Newspackification, which works (and is still necessary for any further customizations!) but this would be a nicer starting point.