Version check for disconnected environments

**EDIT**: For current status, see https://github.com/jamulussoftware/jamulus/issues/1178#issuecomment-919784342

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**Has this feature been discussed and generally agreed?**

Improving version checks has been previously discussed in #1120. As part of that issue reliability has been improved by checking two independent public servers.

This leaves the following unsolved:
- Disconnected environments with no Internet access can still not get update notifications.
- Should we ever have problems with the jamulus.io domain, we'd currently not have a way to signal updates.

**Describe the solution you'd like**
This issue is about deciding (and implementing) if we further want to extend this to checking the currently connected (possibly private) server.

**Describe alternatives that have been considered**

The alternative is not implementing it.

@jamulussoftware/maindevelopers Can you comment or add a reaction if this should be done? (My impression was that @softins did not consider it necessary while @pljones was in favor?)

Start with the current-status comment, then review issue #1120 and linked pull request #1980 to understand what version-check work has already happened. The issue names no files or tests; done would require an agreed decision and implementation for checking the connected private server in disconnected environments.

Written by the indexing model from the issue text.