Add spec to expose a bug finding manager logged in

I've moved the method to the User model in order to make it easier to
test. I'm not sure where it belongs, though.

There was already a failing spec in `spec/features/management_spec.rb`,
but it passed if run standalone because it only failed if previous tests
had already created nine users or more.

app/controllers/management/base_controller.rb

@@ -56,7 +56,7 @@ class Management::BaseController < ActionController::Base
 
     def manager_logged_in
       if current_manager
-        @manager_logged_in = User.find_by(id: session[:manager]["login"].last(1))
+        @manager_logged_in = User.find_by_manager_login(session[:manager]["login"])
       end
     end
 

app/models/user.rb

@@ -329,6 +329,10 @@ class User < ActiveRecord::Base
     where(conditions.to_hash).where(["username = ?", login]).first
   end
 
+  def self.find_by_manager_login(manager_login)
+    find_by(id: manager_login.last(1))
+  end
+
   def interests
     followables = follows.map(&:followable)
     followables.compact.map { |followable| followable.tags.map(&:name) }.flatten.compact.uniq