Add ARIA roles and labels to search, header, logo, mobile menu button, and main content (#1259)

This PR fixes several Axe errors I found while working on https://viewcomponent.org/. I did not see any visual regressions on my deploy, but I'd encourage testing with other sites.

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Co-authored-by: Lindsey Wild <35239154+lindseywild@users.noreply.github.com>

_includes/components/search_header.html

@@ -1,6 +1,6 @@
 {% capture search_placeholder %}{% include search_placeholder_custom.html %}{% endcapture %}
 
-<div class="search">
+<div class="search" role="search">
   <div class="search-input-wrap">
     <input type="text" id="search-input" class="search-input" tabindex="0" placeholder="{{ search_placeholder | strip_html | strip }}" aria-label="{{ search_placeholder | strip_html| strip }}" autocomplete="off">
     <label for="search-input" class="search-label"><svg viewBox="0 0 24 24" class="search-icon"><use xlink:href="#svg-search"></use></svg></label>

_includes/components/sidebar.html

@@ -1,8 +1,8 @@
 <div class="side-bar">
-  <div class="site-header">
+  <div class="site-header" role="banner">
     <a href="{{ '/' | relative_url }}" class="site-title lh-tight">{% include title.html %}</a>
-    <a href="#" id="menu-button" class="site-button">
-      <svg viewBox="0 0 24 24" class="icon"><use xlink:href="#svg-menu"></use></svg>
+    <button id="menu-button" class="site-button" aria-label="Open menu">
+      <svg viewBox="0 0 24 24" class="icon" aria-hidden="true"><use xlink:href="#svg-menu"></use></svg>
     </a>
   </div>
   <nav aria-label="Main" id="site-nav" class="site-nav">

_includes/title.html

@@ -1,5 +1,5 @@
 {% if site.logo %}
-  <div class="site-logo"></div>
+  <div class="site-logo" role="img" aria-label="{{ site.title }}"></div>
 {% else %}
   {{ site.title }}
 {% endif %}