Improve support for Link Roundups in archives

[benlk]

Changes

• Create partials for use in Largo:
  • content-rounduplink.php: no byline, very little in the way of content. This is essentially a rehash of the argolinks partial removed for Remove partials/content-argolinks.php #926.
  • content-roundup.php:
    • displays whole the_content() in archives, as the content will be primarily shortcodes and thus will not be rendered in largo_excerpt.
    • byline is output, buy .visuallyhidden
• In archive.php and category.php, use largo_get_partial_by_post_type for the partial selector, so those can be filtered with largo_partial_by_post_type.

A demonstration of the content-roundup.php partial in the archive:

Generated saved links, with one edited to have a description and link:

## Why

While the Link Roundups plugin can use the filter largo_partial_by_post_type, it can't provide partials that will be loaded by get_template_part( 'partials/content', $partial ) unless we also filter the partials/content part of get_template_part.

So, a compromise:

• Largo keeps the partials
• Link roundups does the filtering ( PR Filter Largo to use the link roundups partials. INN/link-roundups#109 )

To get the /rounduplink/ and /roundup/ archives, all that needs be done is for the site user to resave permalinks after activating the plugin. Both post types are registered with 'has_archive' => true,, so the archives will automatically be available in at those urls.