html: add <meta name="theme-color"> tag.

Make the address bar blue on mobile.

* https://developer.mozilla.org/en-US/docs/Web/HTML/Element/meta/name/theme-color
* https://developers.google.com/web/fundamentals/design-and-ux/browser-customization#meta_theme_color_for_chrome_and_opera

app/views/layouts/default.html.erb

@@ -41,6 +41,7 @@
   <%= tag.meta name: "twitter:description", content: meta_description %>
 
   <%= tag.meta name: "git-hash", content: Rails.application.config.x.git_hash %>
+  <%= tag.meta name: "theme-color", content: "hsl(213, 100%, 50%)" %>
 
   <%= yield :html_header %>
   <%= raw Danbooru.config.custom_html_header_content %>