Calling out to a 3rd party web service like Slack or Mattermost blocks
the request for the current Redmine user. Depending on network latency,
this can result in very slow response on any action that is notified via
`redmine_messenger`.
Instead the backend call should make use of Rails builtin asynchronous
background job queuing (the same as used for Redmine mails), returning
to the user earlier.
We don't specify the actual queue backend for this job, so any Redmine
installation can select an appropiate queue backend by themselves or
simply use the default builtin backends.
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