Just a note that this will not work in Moodle 3.1 because the capability "moodle/site:configview" does not exist there yet.
Which means such a role can't get to the Site admin menu to be able to add a user to a Learning Plan. So, it will not work in most cases, as you would not want to grant site:config to such users, which is far too dangerous.
You'd have to give such a LP assigner some other way to get to site admin menu, which bascially defeats the purpose of having a limited role to do just this one task of assigning LPs to users.
Works as described in 3.2 where this capability exists.