Wordpress WPML database migrations vulnerable to update race-condition

[Firefishy]

The WPML plugin is used on wordpress sites is vulnerable to a race-condition where chef is updating the codebase and a simultaneous web request triggers a partial but incomplete database migration.

WPML marks the migration as complete, but it may not have processed all/any portions of the migration due to the migration codebase files still being deployed.

We should change the deployment code to update the website code + plugins codebase using an atomic action.

[Firefishy]

This could be mitigated by moving the wordpress services to containers per https://github.com/openstreetmap/operations/issues/807