Grant Meditation Access Based on Group Membership

Members of specific groups (like graduate groups for specific programs) should have access to specific exercises that aren't available to everyone else. Take-home exercises would fall into this category as well as unlocking specific focus levels in the Timer section. Take-home exercises should not show up in the general list of exercises for people not in a group that allows access to them. We'll also have to account for people joining a group and refreshing the exercise list based to show new group-related exercises.