Let the Livebook update banner show Nerves Livebook updates instead of regular Livebook updates

[PhillippOhlandt]

When the regular Livebook got a new release, the update banner in Nerves Livebook gets triggered.

Since Livebook and Nerves Livebook don't have matching version numbers and release dates, this is misleading and confusing.

The update check happens in this GenServer: https://github.com/livebook-dev/livebook/blob/main/lib/livebook/update_check.ex

A simple solution would be to make the github release url and the application name configurable using application config so that Nerves Livebook can set its own values.

@fhunleth pointed out that it would also be nice if the user can interact with the update banner to trigger a firmware update.