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/controllers/application_controller.rb

@@ -13,7 +13,7 @@ class ApplicationController < ActionController::Base
   before_action :api_check
   before_action :ip_ban_check
   before_action :set_variant
-  before_action :enable_cors
+  before_action :add_headers
   before_action :cause_error
   after_action :reset_current_user
   layout "default"
@@ -65,8 +65,9 @@ class ApplicationController < ActionController::Base
 
   protected
 
-  def enable_cors
+  def add_headers
     response.headers["Access-Control-Allow-Origin"] = "*"
+    response.headers["X-Git-Hash"] = Rails.application.config.x.git_hash
   end
 
   def api_check

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>