global: fix handling of debug and simplify toolbar
- Fix issue where runserver manage command would not enable the Werkzeug debugger.
- Changes how debug-toolbar is enabled via only DEBUG and EXTENSIONS configuration variables.
- Fixes issue in legacy module that would override app.debug with value of CFG_DEVEL_SITE even though DEBUG had been set in configuration.
NOTE: beware, patch amended by Tibor to use new file names