Exctract Action Web Push

There’s been some interest in helping pull Campfire’s Web Push out into its own gem in [this discussion](https://github.com/basecamp/once-campfire/discussions/78).

@excid3 will be leading this effort. He’s been working on it for the past few weeks with our SIP team and already has [a working prototype](https://github.com/excid3/action_push_web) with generators, a polymorphic subscription model, a Stimulus controller, and VAPID key setup.

If you’re interested in helping out, please chime in here!

Read discussion 78 and the linked action_push_web prototype first to understand the proposed extraction and its current scope. The intended result is a standalone gem covering the mentioned generators, polymorphic subscription model, Stimulus controller, and VAPID key setup; the issue does not name repository files or tests.

Written by the indexing model from the issue text.