Add configuration options for opening external links in new tab (#1360)

I've whipped up a solution that solves #1103. I've added a config option `nav_external_links_new_tab`, which is off by default. When turned on, it'll pop external nav links into a new tab. The idea was borrowed from how [aux_nav.html](https://github.com/just-the-docs/just-the-docs/blob/main/_includes/components/aux_nav.html) does it with `aux_links_new_tab`.

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Co-authored-by: Matt Wang <matt@matthewwang.me>

_includes/components/sidebar.html

@@ -4,7 +4,7 @@
   Results in: HTML for the side bar.
   Includes:
     title.html, components/site_nav.html, nav_footer_custom.html
-  Overwrites: 
+  Overwrites:
     nav_footer_custom.
   Should not be cached, because nav_footer_custom.html might depend on page.
 {%- endcomment -%}
@@ -16,9 +16,9 @@
       <svg viewBox="0 0 24 24" class="icon" aria-hidden="true"><use xlink:href="#svg-menu"></use></svg>
     </button>
   </div>
-  
+
   {% include_cached components/site_nav.html %}
-  
+
   {% capture nav_footer_custom %}
     {%- include nav_footer_custom.html -%}
   {% endcapture %}

_includes/components/site_nav.html

@@ -4,7 +4,7 @@
   Results in: HTML for the site-nav.
   Includes:
     components/nav.html
-  Overwrites: 
+  Overwrites:
     pages_top_size, collections_size, collection_entry,
     collection_key, collection_value, collection.
 {%- endcomment -%}
@@ -22,7 +22,11 @@
     <ul class="nav-list">
       {%- for node in site.nav_external_links -%}
         <li class="nav-list-item external">
-          <a href="{{ node.url | absolute_url }}" class="nav-list-link external">
+          <a href="{{ node.url | absolute_url }}" class="nav-list-link external"
+            {% if node.opens_in_new_tab or (node.opens_in_new_tab == nil and site.nav_external_links_new_tab) %}
+              target="_blank" rel="noopener noreferrer"
+            {% endif %}
+          >
             {{ node.title }}
             {% unless node.hide_icon %}<svg viewBox="0 0 24 24" aria-labelledby="svg-external-link-title"><use xlink:href="#svg-external-link"></use></svg>{% endunless %}
           </a>