luci-mod-network: merge hosts with dns settings

The current location for "Network > Hostnames" is confusing.
It is provided by Dnsmasq and partly overlaps with static leases.

Merge "Hostnames" with "Network > DHCP and DNS" as an extra tab.
Also add a simple explanation of the use case.

Signed-off-by: Vladislav Grigoryev <vg.aetera@gmail.com>
[Fix typo in label description string]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>