Filter template hierarchy for embed templates

[mmirus]

Add embed to the template hierarchy filters in Sage, so that the embed template can be overridden from the views folder.

Closes #2142

Once this is merged, you can copy wp-includes/theme-compat/embed.php to views and adapt it however you want, just like any template file in Sage. Better yet, create views/embed.blade.php , with something like:

@include('partials.header-embed')

@if (have_posts())
  @while (have_posts()) @php the_post() @endphp
    @include('partials.embed-content')
  @endwhile
@else
  @include('partials.embed-404')
@endif

@include('partials.footer-embed')

The above assumes that you've also added embed-content, embed-404, and footer-embed templates to the partials folder, but you don't have to if you don't need to customize them.