Convert member role to learner role

[heyheyjp]

Users of any type can have membership in a chapter. We need to be able to distinguish learner members from non-learner members. We've been using member in idm as a synonym for learner, though echo has a more generic concept of membership. To clarify this distinction and also allow echo to more clearly distinguish learners from non-learners, we'll convert the member role to learner.