Extract constant to define investments per page

That way it's easier to stub in tests. It makes it easier to customize
CONSUL to show a different number of investments per page as well.

app/controllers/budgets/investments_controller.rb

@@ -6,6 +6,8 @@ module Budgets
     include FlagActions
     include ImageAttributes
 
+    PER_PAGE = 10
+
     before_action :authenticate_user!, except: [:index, :show, :json_data]
 
     load_and_authorize_resource :budget, except: :json_data
@@ -37,7 +39,7 @@ module Budgets
     respond_to :html, :js
 
     def index
-      @investments = investments.page(params[:page]).per(10).for_render
+      @investments = investments.page(params[:page]).per(PER_PAGE).for_render
 
       @investment_ids = @investments.pluck(:id)
       @investments_map_coordinates = MapLocation.where(investment: investments).map(&:json_data)