luci-mod-network: interfaces: restructure DHCPv6 and IPv6 RA options

luci-mod-network: interfaces: restructure DHCPv6 and IPv6 RA options

 - Condense overly large IPv6 RA/DHCPv6 description texts and get rid of most embedded markup

 - Switch ra/ndp/dhcpv6 mode selections to rich dropdown lists and move extended choice
   descriptions next to the selection options

 - Drop ndproxy_static option which has been removed from odhcpd long ago

 - Add format validations to all text input fields

 - Add ability to configure master/relay modes for non-static interfaces (#2998)

 - Move extended RA configuration options into a new tab

 - Prevent enabling master mode on multiple interfaces

 - Prevent enabling ra/dhcpv6 server mode on non-static or master interfaces

 - Drop ra_management in favor to ra_flags option (#5083)

 - Add support for dns_service option

 - Read current effective IPv6 MTU and hop limit placeholder values from procfs

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(cherry picked from commit 3fbd4338846e8229935b54256a3a541a3e15d8bd)