FIX: Better handling of Group model state (#8356)

The group card and group members page were affecting each other and were
leaking members list and the query parameters which led to bad UX
experience and sub-optimal performance (client made more queries because
it was loading fewer members).

This commit refactors the group model to make it more consistent, remove
dead code, move error handling outside of model.

test/javascripts/acceptance/group-requests-test.js.es6

@@ -37,6 +37,7 @@ acceptance("Group Requests", {
           is_group_user: true,
           is_group_owner: true,
           is_group_owner_display: true,
+          can_see_members: true,
           mentionable: false,
           messageable: false
         },