gluon-client-bridge, gluon-mesh-batman-adv-core: switch roles of br-client and local-node interfaces

MAC and IP addresses are switched. This makes the gluon-client-bridge
package more useful for different routing protocols that don't need a
unique address on the client bridge.

As a side effect, gluon-radvd is now using the next-node address, which had
been considered before, but was dismissed to avoid having gluon-radvd
depend on gluon-next-node and gluon-mesh-batman-adv. This will be useful
for announcing default routes via gluon-radvd.

One downside is that this introduces a minor dependency on batman-adv in
gluon-respondd: the hotplug script that checked for the client interface
before will now check for local-node. This doesn't really matter: for mesh
protocols without a local-node interface, the check will do nothing (which
makes sense, as there is no interface to bind to for mesh-wide respondd).

The next-node interface may be useful with IPv4 only or even just a MAC
address. This prepares merging gluon-next-node into gluon-batman-adv-core.

Because we unconditionally appended `-i br-client` to the command line of
respondd, it wasn't restarted when br-client changed state. Now, we use a
jsonfilter expression on the network.interface dump data, similar to how the
other interface names are generated, and only add the interface to the argument
list if it is up.

Fixes #941

The interfaces were accidentially switched when the CPE210 was separated
from the CPE510 code.

If cookies are disabled, the Statuspage only displays an empty ("Not connected")
This checks if the localStorage API is available and working and only uses it in this case
Also allows better backwards compatibility.

Fixes #886

If next_node.ip4 is not set or gluon-next-node is not used, prefix4 will
now be optional as well.

None of them are used by modern browsers anymore, so it's safe to remove them.

Fixes #834

Fixes #842

Users may have defined additional mesh interfaces. Properly migrate these
to avoid subtly breaking the network config (and make them ready for new
mesh protocols).

gluon-client-bridge: add LAN interfaces to client bridge by default (instead of doing this in the batman-adv-specific scripts)

This allows to weaken the gluon-mesh-batman-adv-core dependency of
gluon-luci-portconfig to gluon-client-bridge.

This new protocol will allow configuring mesh interfaces independently of
the used mesh protocol.

Switch to:

1. WAN
2. LAN
3. Mesh VPN

As WAN and LAN are setup in gluon-mesh-batman-adv-core (and will be moved
to gluon-core), while the mesh VPN has its own package, giving WAN and LAN
the first indices is preferable.

Fixes #854

Fixes #414

Allows weird release numbers, models and site names, and prepares support
for pretty hostnames.

and add documentation

Fixes #862

Just like we enabled multicast snooping on the batman-adv client bridge
again, let's do the same for the WAN side.

With one exception: The IGMP/MLD querier is kept disabled to avoid
becoming too "bossy"/"noisy" on a foreign network. The main router on
the WAN side should perform querying and by that enable
IGMP/MLD/snooping if it considers this appropriate there.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>