Add Lunr Search (#1353)

* Add `search.html` layout and JavaScript
* Move `{{ content }}` before search input
* Add spaces for consistency

Close #1011

_layouts/search.html

@@ -0,0 +1,30 @@
+---
+layout: default
+---
+
+{% if page.header.overlay_color or page.header.overlay_image or page.header.image %}
+  {% include page__hero.html %}
+{% endif %}
+
+{% if page.url != "/" and site.breadcrumbs %}
+  {% unless paginator %}
+    {% include breadcrumbs.html %}
+  {% endunless %}
+{% endif %}
+
+<div id="main" role="main">
+  {% include sidebar.html %}
+
+  <div class="archive">
+    {% unless page.header.overlay_color or page.header.overlay_image %}
+      <h1 class="page__title">{{ page.title }}</h1>
+    {% endunless %}
+
+    {{ content }}
+    
+    <input placeholder="Search..." type="search" id="search" class="search-input">
+    <div id="results"></div>
+
+
+  </div>
+</div>