luci-mod-network: add tags tab

luci-mod-network: add tags tab

For use with dnsmasq tags

Note that a MAC tab is possible but the same functionality
is present in the leases tab which handles MACs and tags.
A MAC tab has a 1:1 tag:MAC relationship, whereas the leases
has a many:many relationship.

The dnsmasq init file needs updating to use 'tag' in
place of 'networkid', which is an older legacy format still
understood by dnsmasq, but all documentation uses 'tag'.

tag names shall not match:
- network devices/interfaces
- service names

Three dnsmasq reserved tag names are:
- known
- !known
- known-othernet

Tag names can be prepended with '!' to invert their usage.

Closes #7178

Signed-off-by: Paul Donald <newtwen+github@gmail.com>