luci-app-mwan3: fix legacy uci api usage

luci-app-mwan3: fix legacy uci api usage

Explicitely require libuci-lua in model classes that use legacy /var/state
cursor handling.

Also add a specific dependency on libuci-lua to the luci-app-mwan3
Makefile in preparation of the upcoming default removal of libuci-lua.

Finally fix the post data dispatching on the notification tab, see #1722
for reference.

Fixes #1726.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>