Updated installation instructions in README.

Fixed #19741.
Upgrade Django. Switch to the new project layout.
Rename ux project for consistency.
Move apps outside of the django_website package.
Remove obsolete URL that references a non-existing template.
Remove obsolete django_website.shortcuts.render.
Remove obsolete code that isn't even importable.
Move the templates directory at the toplevel.
Setup modern static files handling.
Move BASE one level up to simplify settings.
Move the docs recent release context processor to the docs app.
Remove legacy WSGI handlers.

PYTHONPATH will be set in mod_wsgi's conf.
Prevent the docs context processor from crashing on an empty database.
Add tests for the svntogit app.