Add warning about default routes

[untitaker]

I hope this puts a lot of questions about SIM cards to rest. I found
that the warning also sometimes applies to "dead" SIM cards which have
expired a long time ago.

Run busybox ip route to determine whether the device has an active SIM
card. That command has been manually tested on Moxee, Orbic and TP-Link.
It's prefixed with busybox because that makes it more likely it would
work on UZ801, though it wasn't tested there. If the command invocation
fails, the alert is suppressed and a warning is logged.

The command is only run once on pageload. It could've been part of the
status endpoint, but then the UI would poll it way too often.

Fix #345

[wgreenberg]

i like this feature idea a lot, but i wonder if there's a more portable solution available. i ran strace ip route locally, and looks like it gets all of that info via the netlink socket API. i found a project that looks pretty promising for using that API within rust called neli, and here's an example that uses it to get link IPs: https://github.com/jbaublitz/neli/blob/main/examples/getips.rs

that said, i wager this is good enough for now, so i'll go ahead and merge and open an issue to use the more portable version. thanks untitaker!

[kmille]

A bit feedback: It's a bit annoying dismissing the pop up after each reload. It would be nice if this pop up is only shown once (after a reboot of the device).

[untitaker]

I'll rework the warnings next release if more people complain. to be honest I was thinking to even make the warning non-dismissable since the device is plainly unfit for purpose if it appears.