MT#3977 Implement line/key handling for devices.

This is part 1 to define keys/lines on a device, to be assigned to
subscribers.
MT#3977 Prepare line/key in PBX field dev.
MT#3977 populate PBX field dev form on prof change
MT#3977 Handle new lines added by user in UI.
MT#3977 Implement backend for multi-line PBX conf

This makes it possible to configure multiple lines (private, shared,
blf) and attendant console keys (blf) on a field device and generate the
proper config for it.

It does not yet support editing such a field device.
MT#3977 Implement editing of PBX field device.
MT#3977 Display stationname/keyid in PBX field dev
MT#3977 Add option in status msg to manually sync.
MT#3977 properly handle firmware upgrade.
MT#3977 provide example config template SPA5xx
MT#3977 Fix accordion expand/collapse toggle.