ip addresses: add ip address show page.

* Add IP address show page at /ip_addresses/1.2.3.4.
* Add "Details", "Ban IP", "Ban User" options to the /ip_addresses listing.

config/danbooru_default_config.rb

@@ -519,6 +519,12 @@ module Danbooru
       nil
     end
 
+    # API key for Google Maps. Used for embedding maps on IP address lookup pages.
+    # Generate at https://console.developers.google.com/apis/credentials
+    def google_maps_api_key
+      nil
+    end
+
     # Cloudflare API token. Used to purge URLs from Cloudflare's cache when a
     # post is replaced. The token must have 'zone.cache_purge' permissions.
     # https://support.cloudflare.com/hc/en-us/articles/200167836-Managing-API-Tokens-and-Keys