Switch to a network-wide option

Last PR from me for a while I think. You should tag a release on wp.org :)

I think airplane mode should be a network-wide option when using Multisite. Currently the unintended effect is that the setting doesn't persist across sites within a network.

Switching from the `*_option()` functions to the `*_site_option()` functions solves this easily (they are just wrappers for their `*_option()` counterpart on single site installs).