Feature: Incorporate site search into masthead (#1383)

* Integrate search into masthead
* Fix cutoff descenders in archive article titles
* Remove search page from `/test` site
* Enable masthead search
* Remove dedicated search page
* Fix masthead search form padding
* Improve insertion of search content
* Speed up page transition
* Add fade transition to search content
* Rename visibility class names
* Add `site.search` to _config.yml
* Document site search feature
* Update CHANGELOG and history

_layouts/default.html

@@ -19,7 +19,15 @@
     {% include browser-upgrade.html %}
     {% include masthead.html %}
 
-    {{ content }}
+    <div class="initial-content">
+      {{ content }}
+    </div>
+
+    {% if site.search == true %}
+      <div class="search-content">
+        {% include search_form.html %}
+      </div>
+    {% endif %}
 
     <div class="page__footer">
       <footer>