luci-mod-network: network/routing support for pbr

Provide comprehensive routing configuration:
* Rename the "Network > Routes" page to "Network > Routing".
* Unify sorting for the "Status" and "Network" menus.
* Add the tabs "IPv4 Rules" and "IPv6 Rules" to the "Routing" page.
* Provide configuration for IPv4 and IPv6 routing rules.
* Consolidate routing configuration and terminology for IPv4 and IPv6.

Policy-based routing is an increasingly popular problem.
Netifd natively supports policy-based routing:
* Interface-specific options "ip4table" and "ip6table".
* Routing rules using the "rule" and "rule6" sections.
LuCI is missing configuration for routing rules.

Signed-off-by: Vladislav Grigoryev <vg.aetera@gmail.com>