Include git hash in http headers and <meta> tags.

* Add X-Git-Hash http header.
* Add `<meta name="git-hash" content="<hash>">` meta tag.
* Include full hash in development mode.

app/views/layouts/default.html.erb

@@ -77,6 +77,8 @@
   <%= tag.meta name: "twitter:title", content: page_title %>
   <%= tag.meta name: "twitter:description", content: meta_description %>
 
+  <%= tag.meta name: "git-hash", content: Rails.application.config.x.git_hash %>
+
   <%= yield :html_header %>
   <%= raw Danbooru.config.custom_html_header_content %>
 </head>