Mark front page announcements as <aside> elements

This matches the HTML used on the rest of the site. It helps avoid
search engines treating the aside as the primary content of the web
page.

Author: Tim Bannister

layouts/partials/frontpage-announcement.html

@@ -1,6 +1,6 @@
 {{ if .Page.Param "announcement" }}
 <section lang="en" id="fp-announcement" style="background-color:{{ .Page.Param "announcement_bg" }}">
-  <main>
+  <aside>
     <div class="content announcement main-section">
 
       <h3>
@@ -9,6 +9,6 @@ <h3>
       <p>{{ T "announcement_message" | markdownify }}</p>
 
     </div>
-  </main>
+  </aside>
 </section>
 {{ end }}

static/css/announcement.css

@@ -33,7 +33,7 @@
   min-height: 30vh;
 }
 
-#fp-announcement main {
+#fp-announcement aside {
   padding-top: 125px;
   padding-bottom: 35px;
 }