Move group member title to header

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/views/groups/group_members/index.html.haml

 - page_title "Members"
+- header_title group_title(@group, "Members", group_group_members_path(@group))
 - show_roles = should_user_see_group_roles?(current_user, @group)
 
-%h3.page-title
-  Group members
 - if show_roles
   %p.light
     Members of group have access to all group projects.
     Read more about permissions
     %strong= link_to "here", help_page_path("permissions", "permissions"), class: "vlink"
 
-%hr
 
 .clearfix.js-toggle-container
   = form_tag group_group_members_path(@group), method: :get, class: 'form-inline member-search-form'  do