Connectivity check should use our URL

[cassidyjames]

(Not 100% sure this is the right repo; we can move it if needed.)

What Happened

I thought we were already using https://elementary.io/capnet-assist for the connectivity check, but a user is reporting it's hitting Google's version (e.g. for Android and Chrome OS) because it's the default in the Ubuntu config.

Expected Behavior

We should ensure we're hitting our own server here since it's open source and hosted by us instead of Google.

[danirabbit]

I'm not sure this is something for patches as much as maybe for default-settings? We're currently shipping network-manager-config-connectivity-ubuntu. So if we wanted to provide our own config, we could use provides/conflicts instead of a patch