Add statistics route, permissions, and sidebar entry

- /statistics route and PagePaths.statistics
- Permission sets: viewer and admin can access /statistics
- Sidebar link with can_access_page check
- Plug and sidebar tests updated

lib/mv_web/page_paths.ex

@@ -9,6 +9,7 @@ defmodule MvWeb.PagePaths do
   # Sidebar top-level menu paths
   @members "/members"
   @membership_fee_types "/membership_fee_types"
+  @statistics "/statistics"
 
   # Administration submenu paths (all must match router)
   @users "/users"
@@ -31,6 +32,9 @@ defmodule MvWeb.PagePaths do
   @doc "Path for Membership Fee Types index (sidebar and page permission check)."
   def membership_fee_types, do: @membership_fee_types
 
+  @doc "Path for Statistics page (sidebar and page permission check)."
+  def statistics, do: @statistics
+
   @doc "Paths for Administration menu; show group if user can access any of these."
   def admin_menu_paths, do: @admin_page_paths