Allow platform admins to clear cached broadcasts

When we add a new property to the broadcast model, we need to delete any
cached broadcasts from Redis that are missing the new property. So this
adds an option to do this to the cache page in platform admin.

I’ve also tried to make it more obvious what the magic numbers in the
test fixture are doing.

app/main/views/platform_admin.py

@@ -432,6 +432,9 @@ def clear_cache():
             'live-service-and-organisation-counts',
             'organisation-????????-????-????-????-????????????-name',
         ]),
+        ('broadcast', [
+            'service-????????-????-????-????-????????????-broadcast-message-????????-????-????-????-????????????',
+        ]),
     ])
 
     form = ClearCacheForm()