[Core] Warn when OTA upgrading relays

Plugs and other relay devices will reboot as part of an OTA update. If your plug is powering something important (e.g. expensive audio equipment or a server), it will usually turn off for a little bit as the firmware resets. This isn't intuitive. We should warn about this in the OTA info.

Side note: our warnings right now are hard-coded strings. Is there a way to make them translatable?