FEATURE: Add order logic to admin users controller

Added order and direction parameters for sorting admin user pages. This
commit only includes backend api changes.

https://meta.discourse.org/t/make-admin-users-list-sortable-suggestion/47649

Now you can pass in `order` and `asc` parameters to the
`/admin/users/list/<query>.json` endpoint.

Example:

`/admin/users/list/active.json?&order=post_count` which defaults to desc

and

`/admin/users/list/active.json?order=post_count&asc=true`

lib/admin_user_index_query.rb

@@ -11,15 +11,28 @@ class AdminUserIndexQuery
   attr_reader :params, :trust_levels
 
   def find_users(limit=100)
-    find_users_query.includes(:user_stat).limit(limit)
+    find_users_query.limit(limit)
   end
 
   def count_users
     find_users_query.count
   end
 
-  def self.orderable_columns
-    %w(created_at days_visited posts_read_count topics_entered post_count trust_level)
+  def self.orderable_user_columns
+    %w(created_at trust_level last_emailed_at last_seen_at username email)
+  end
+  
+  def self.orderable_stat_columns
+    %w(days_visited posts_read_count topics_entered post_count time_read)
+  end
+
+  def custom_direction
+    asc = params[:asc]
+    asc.present? && asc ? "ASC" : "DESC"
+  end
+
+  def pg_coalesce(column)
+    "COALESCE(#{column}, to_date('1970-01-01', 'YYYY-MM-DD'))"
   end
 
   def initialize_query_with_order(klass)
@@ -28,20 +41,31 @@ class AdminUserIndexQuery
     custom_order = params[:order]
     if custom_order.present? &&
       without_dir = custom_order.downcase.sub(/ (asc|desc)$/, '')
-      if AdminUserIndexQuery.orderable_columns.include?(without_dir)
-        order << custom_order
+      if AdminUserIndexQuery.orderable_user_columns.include?(without_dir)
+        without_dir = (without_dir == "last_seen_at") ? pg_coalesce("last_seen_at") : without_dir
+        without_dir = (without_dir == "last_emailed_at") ? pg_coalesce("last_emailed_at") : without_dir
+        order << "#{without_dir} #{custom_direction}"
+      end
+      if AdminUserIndexQuery.orderable_stat_columns.include?(without_dir)
+        order << "user_stats.#{without_dir} #{custom_direction}"
       end
     end
+    
+    if !custom_order.present?
+      if params[:query] == "active"
+        order << "#{pg_coalesce("last_seen_at")} DESC"
+      else
+        order << "users.created_at DESC"
+      end
 
-    if params[:query] == "active"
-      order << "COALESCE(last_seen_at, to_date('1970-01-01', 'YYYY-MM-DD')) DESC"
-    else
-      order << "users.created_at DESC"
+      order << "users.username"
     end
 
-    order << "users.username"
-
-    klass.order(order.reject(&:blank?).join(","))
+    if params[:stats].present? && params[:stats] == false
+      klass.order(order.reject(&:blank?).join(","))
+    else
+      klass.includes(:user_stat).order(order.reject(&:blank?).join(","))
+    end
   end
 
   def filter_by_trust