LTI Consumer should send more user roles

Current implementation makes bad assumptions to try to determine roles that are sent to LTI Tool Provider, like this one:

 if (has_capability('moodle/course:manageactivities', $coursecontext)) {

     array_push($roles, 'Instructor');

 } else {

     array_push($roles, 'Learner');

 }

A better way to deal with this would use "role archetypes" and hardcode a relation between "archetype" and the corresponding "lti role".

Also I believe that "every role" should be passed, but prepending a Moodle's specific namespace. LTI specification doesn't prohibit us from using custom roles.