export: Create REALM_EXPORTED audit log for exports via shell.

Earlier, we were creating RealmAuditLog with REALM_EXPORTED
event_type when export of public data took place via organization
settings panel.

We were not creating the audit log when the export was executed
via shell i.e './manage.py export'.

This commit creates the audit log in that case too. It will
help during import to distinguish readily between imports
from another Zulip server vs imports from another product.

zerver/lib/export.py

@@ -2418,9 +2418,15 @@ def export_realm_wrapper(
 
 
 def get_realm_exports_serialized(user: UserProfile) -> List[Dict[str, Any]]:
+    # Exclude exports made via shell. 'acting_user=None', since they
+    # aren't supported in the current API format.
+    #
+    # TODO: We should return those via the API as well, with an
+    # appropriate way to express for who issued them; this requires an
+    # API change.
     all_exports = RealmAuditLog.objects.filter(
         realm=user.realm, event_type=RealmAuditLog.REALM_EXPORTED
-    )
+    ).exclude(acting_user=None)
     exports_dict = {}
     for export in all_exports:
         export_url = None