Rename deviceName config key to device_name

[lorthirk]

To stay coherent with the other configuration keys being snake_case, I renamed deviceName to device_name. It should break the actual configured value, but setting it again should be fine.

[v1nc]

I support this PR, but keep in mind that it will reset the visible device name for every user who updates. Maybe we should manually set device_name to the value of deviceName at the first start after update?