common/highlightjs: Turn some global variables into functions

The _hljs_languages* global variables content rely on reading a JSON file
located through Django static finders.

As a side effect, when the module is imported, the JSON file is located
then read.

When the DJANGO_SETTINGS_MODULE environment variable is not set, the
module import will fail as the JSON file could not be located.

So avoid to execute that JSON loading code when importing the module
by turning the global variables into functions.

The purpose is to avoid import errors when running sphinx autodoc.