settings: define sqlite memory backend as default (#18177)

It's not used anyway, this avoids an unnecessary file and make it
possible to run multiple instances (useful for debugging live servers,
with one instance tied to uwsgi and a debugging instance started with
"runserver").

wcs/settings.py

@@ -16,7 +16,7 @@ MANAGERS = ADMINS
 DATABASES = {
     'default': {
         'ENGINE': 'django.db.backends.sqlite3',
-        'NAME': os.path.join(PROJECT_PATH, 'wcs.sqlite3'),
+        'NAME': ':memory:',
     }
 }