Add base directory support

- Base directory can be changed in config.py from the default / to, for example, /taiko-web/
  - See tools/nginx_subdir.conf for an example nginx configuration with a base directory
- Custom error pages can be used, they can be set in config.py

config.example.py

@@ -1,3 +1,6 @@
+# The base URL for Taiko Web, with trailing slash.
+BASEDIR = '/'
+
 # The full URL base asset URL, with trailing slash.
 ASSETS_BASEURL = '/assets/'
 
@@ -7,6 +10,11 @@ SONGS_BASEURL = '/songs/'
 # Multiplayer websocket URL. Defaults to /p2 if blank.
 MULTIPLAYER_URL = ''
 
+# Send static files for custom error pages
+ERROR_PAGES = {
+    404: ''
+}
+
 # The email address to display in the "About Simulator" menu.
 EMAIL = None