Fix TemplateOverride.save() bug

The code was assuming that the Django internal variable
template_source_loaders had been initialized (in order to clear any Django
template caches).  This is not always the case, leading to an error
(sometimes on a live site, always from the shell) when saving a template
override.  Checking whether the variable is a list fixes the problem.
(cherry picked from commit 09187958a87f0dd50e5c2d04187d70c141873789)