Django ignores the value of the TEMPLATE_* settings if TEMPLATES is also
set, which is confusing for users following older tutorials. This change
adds a system check that warns if any of the TEMPLATE_* settings have
changed from their defaults but the TEMPLATES dict is also non-empty.
Removed the TEMPLATE_DIRS from the test settings file; this was marked
for removal in 1.10 but no tests fail if it is removed now.
If `'loaders'` is present in the `TEMPLATES` options together with
`APP_DIRS` set to `True`, the template engine raises an exception. This
conflict is now detected by the system check templates.E001.
This is the result of Christopher Medrela's 2013 Summer of Code project.
Thanks also to Preston Holmes, Tim Graham, Anssi Kääriäinen, Florian
Apolloner, and Alex Gaynor for review notes along the way.
Also: Fixes#8579, fixes#3055, fixes#19844.