Add setting to ignore VPNs at connection detection (bitfireAT/davx5#356)

* Add setting to ignore VPNs at connection detection

* Minor changes

- move methods to ConnectionUtils to keep SyncWorker class compact
- always use "ignore VPNs" as Boolean
- other minor changes

* Show ignore VPNs setting only below api lvl 23

* Change strings

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Co-authored-by: Ricki Hirner <hirner@bitfire.at>