server, pages, tasks (brython): common_scripts: - scripts common to all pages can now be put in root `_browser` dir - `scripts` list is not set in `dyn_data`'s `template` anymore, but directly in a root `scripts` key (because it's not data which are sent verbatim to the template renderer) - if an `__init__.py` script is found in root `_browser` dir, brython task will add it to template scripts.