luci-mod-network: tools.network: handle bridge config and vlan corner cases

luci-mod-network: tools.network: handle bridge config and vlan corner cases

 - Disable interface-level bridging if a corresponding br-$name bridge
   already exists as device declaration

 - Exempt wireless interfaces from bridge port configuration, they can only
   be attached indirectly through "option network"

 - Consider bridge ports from both "option ifname" in interface/device
   sections and from "option ports" in bridge-vlan ones

 - Small fixes for rendering quirks

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