Rewrite Megaphone server using Actix framework

[data-sync-user]

Currently the megaphone server uses an older version of Rocket as it’s framework, which requires rust nightly. This was fine when it was originally created 6 years ago, but a lot of things changed, and now we would like to centralize to the actix framework.

Megaphone is in long maintenance mode, and this would be a reasonably significant effort, but would improve overall security and long term support for megaphone.

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