Add deploy to fly on PR

[tverum]

Followed the guide of this link.
Basically, on a PR open|sync|close|reopen -> will publish a review app to fly.io.

Will be available at https://pr---.fly.dev

Currently unsure if it actually published on a commit to master

[tverum]

Need to add the Fly token to the github secrets still.
@bart6114 could you do that.

Otherwise I can't debug further.

[bart6114]

PR deploy works now. Now let's also do an actual deploy when on main.

Should we not make this a simple switch? If on main, then do a regular deploy; if on PR, then do a PR deploy.

I'd also tend to think we should either:

1. Put this as another job in the CI workflow, or
2. Let the CI workflow trigger this deployment workflow (which, in my opinion, feels a bit hacky in GitHub)

It feels weird to have something deployed without tests. What do you think?

[bart6114]

closing in favour of a simple deploy via fly ui