Make role dropdown on group admin area consistent

What does this MR do?

Change the group role dropdown on group admin area to new style.

Screenshots

Before	After

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/28921, https://gitlab.com/gitlab-org/gitlab-ce/issues/35424

changed the description

added user management label

assigned to @cperessini

@winh LGTM. Approving.

assigned to @winh

approved this merge request

mentioned in issue #28921 (moved)

assigned to @iamphill

merged

mentioned in commit d316a8c0