Enable whitelist for networks to sync in.

[lenucksi]

This app should allow to only attempt to sync when inside a given list of networks. This would be very useful when the configured Nextcloud instance is not reachable from the public internet but only within a given network.

Steps to reproduce

1. Configure app to sync to an account whose server is only reachable from a specific network
2. Attempt to sync outside of said network
3. Repeated error message that server is not found.

Expected behaviour

A configuration dialog should enable restrict syncing to a number of SSIDs.
After that, while not inside on of those networks no sync should occur and thus no error messages on missing servers should show up.

Actual behaviour

s.o.

[nerzhul]

Currently you can filter by connection type. It's not easy to implement SSID filtering but it can be an option yes