Cache organisations, not just domains in Redis

This should make the ‘All organisations’ page load a lil’ bit quicker.
Still worth caching the domains separately so the response is smaller
when we only care about domains. This is because the code that uses the
domains is part of the sign up flow, so it’s really important that it’s
snappy.

app/main/views/platform_admin.py

@@ -417,7 +417,12 @@ def clear_cache():
         ('letter_branding', [
             'letter_branding',
             'letter_branding-????????-????-????-????-????????????',
-        ])
+        ]),
+        ('organisation', [
+            'organisations',
+            'domains',
+            'live-service-and-organisation-counts',
+        ]),
     ])
 
     form = ClearCacheForm()