luci-base: network.js: recognize DSA ports and bridge VLANs

Use the newly introduced devtype attribute for more robust network device
type detection. This also allows us to easily recognize DSA ports.

Furthermore, synthesize VLAN devices declared by uci bridge-vlan sections,
similar to how it is done for legacy swconfig switch_vlan ones.

Finally implement a new Network.Device.getParent() method to use the newly
available "parent" attribute to resolve the base device of DSA ports or
VLAN devices.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>