Support ActionCable in-app mode
This is the omnibus MR for the changes introduced in gitlab!35225 (merged)
This adds support for enabling ActionCable in app mode where we don't start the ActionCable service, but just run the existing Puma server with ACTION_CABLE_IN_APP=true
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We also moved the worker pool size setting to an environment variable because there were issues with it being in gitlab.yml
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Technically, we could also support this with Unicorn, but I left it out of this MR for now since this is experimental anyway.
Edited by Heinrich Lee Yu