Manually correct social media external links

Solution options here were:

(1) spend a long time figuring out why the Markdown renderer that's supposed to handle links isn't seeing the "18F on {platform}" links in the social media sidebar, or
(2) manually mark them external.

I opted for (2) just to get the migration done for now. I've opened another issue to handle this more systematically — our renderers are in rough shape.

data/social_media.yml

@@ -1,18 +1,27 @@
 -
   platform: GitHub
   link: https://github.com/18F
+  html:
+    class: usa-link--external
+    rel:   noreferrer
   image:
     light: assets/img/social-icons/svg/github-lightest.svg
     dark: assets/img/social-icons/svg/github-darkest.svg
 -
   platform: Twitter
   link: https://twitter.com/18F
+  html:
+    class: usa-link--external
+    rel:   noreferrer
   image:
     light: assets/img/social-icons/svg/twitter-lightest.svg
     dark: assets/img/social-icons/svg/twitter-darkest.svg
 -
   platform: LinkedIn
   link: https://www.linkedin.com/company/gsa18f
+  html:
+    class: usa-link--external
+    rel:   noreferrer
   image:
     light: assets/img/social-icons/svg/linkedin-lightest.svg
     dark: assets/img/social-icons/svg/linkedin-darkest.svg

templates/social-media.html

@@ -4,7 +4,8 @@ <h2 class="font-sans-lg text-bold margin-bottom-2">Follow 18F</h2>
     {% for item in social_media %}
     <li class="margin-bottom-2">
       <a href="{{ item.link | url }}"
-         class="usa-link--alt"
+         class="usa-link--alt {{ item.html.class }}"
+         rel="{{ item.html.rel }}"
          >
          {% image_with_class item.image.light "maxw-3 margin-right-2 text-sub" "" %}
          {%- unless item.platform == "RSS feed" -%}18F on {% endunless %}{{ item.platform }}