Have a maintenance mode on NLDS web server #139
Description
It would be useful to have a mechanism to drain transactions on NLDS.
We have the benefit that the consumers and web-server are two different deployments.
We can change the web-server to have a mode that is "maintenance", and the client to print a message to the user if they use the client while the server is in maintenance mode.
This would stop any extra transactions being submitted to the queues while maintenance is ongoing.
I'd propose setting an environment variable in the helm chart, which the web-server picks up, and returns a sensible error code to the client when it is contacted.