views: update seo title and description of post index page.

Change the title of the post index page to look like this:

    "Danbooru: Anime Image Board" (for the front page)
    "Kantai Collection Art | Danbooru" (for a tag search)

Change the meta description of the front page to look like this:

    Danbooru is the original anime image 'booru. Find over 3.75 million
    anime pictures categorized by over 100 million tags.

Change the meta description for a tag search to look like this:

    Find over 37,168 Azur Lane images on Danbooru. Azur Lane (碧蓝航线)
    (벽람항로) is a Chinese shipgirl-themed side-scrolling shoot 'em up
    mobile game developed by Shanghai Manjuu and Xiamen Yongshi...

app/views/layouts/default.html.erb

@@ -48,7 +48,7 @@
     "@context": "http://schema.org",
     "@type": "WebSite",
     "name": "<%= Danbooru.config.app_name %>",
-    "alternateName": "<%= Danbooru.config.description %>",
+    "alternateName": "Anime Image Board",
     "url" : "<%= root_url %>"
   }
   </script>
@@ -76,6 +76,11 @@
   </script>
   <%= tag.meta name: "twitter:title", content: page_title %>
   <%= tag.meta name: "og:title", content: page_title %>
+  <% if meta_description.present? %>
+    <%= tag.meta name: "description", content: meta_description %>
+    <%= tag.meta name: "og:description", content: meta_description %>
+    <%= tag.meta name: "twitter:description", content: meta_description %>
+  <% end %>
   <%= yield :html_header %>
   <%= raw Danbooru.config.custom_html_header_content %>
 </head>