actions: Add do_set_realm_user_default_setting.

This commit adds do_set_realm_user_default_setting which
will be used to change the realm-level defaults of settings
for new users.

We also add a new event type "realm_user_settings_defaults"
for these settings and a "realm_user_settings_default" object
in '/register' response containing all the realm-level default
settings.

zerver/models.py

@@ -3639,6 +3639,7 @@ class AbstractRealmAuditLog(models.Model):
     REALM_SPONSORSHIP_PENDING_STATUS_CHANGED = 213
     REALM_SUBDOMAIN_CHANGED = 214
     REALM_CREATED = 215
+    REALM_DEFAULT_USER_SETTINGS_CHANGED = 216
 
     SUBSCRIPTION_CREATED = 301
     SUBSCRIPTION_ACTIVATED = 302