Extend configuration for update check to include version

[a-maze-d]

The Livebook.Config.app_version/0 is hard coded and can't be changed. This hard coded version corresponds more to the library version and not to the app version (and therefore can't be overwritten).

This PR introduces that distinction by introducing a new Livebook.Config.livebook_version/0 function and make the Livebook.Config.app_version/0 overwritable through config/config.exs.

You can find a more information here

[github-actions]

Uffizzi Preview deployment-56291 was deleted.

[josevalim]

In this case I would prefer then to change the URL configuration to be a tuple, so it holds both the version and the URL. For everything else we keep the version as is, otherwise it can lead to confusion (for example, someone reports a bug for LV 0.13 but it is actually the nerves Livebook version and so on).

[a-maze-d]

That make sense, but we don't have a URL, so I guess you mean the github_release_repo. Should we change it to a github_release_repo_and_version. Not 100% happy with that. Let me give it some more thoughts.

[josevalim]

Yes, we can call it github_release_info.

[a-maze-d]

I reverted my previous commit and followed your recommendation. What do you think?

[a-maze-d]

I did run the tests locally and they do pass (but they do seem to be flaky 😞 and I have to run them several times. I guess that's the reason why the GH action failed.