Add role_invoices

This new role allows users to create invoices without role_finance. Users
can then only modify their own created invoices until somebody with the
role_finance sets the paid_on value.

db/migrate/20160217194036_add_role_invoices_to_group.rb

@@ -0,0 +1,5 @@
+class AddRoleInvoicesToGroup < ActiveRecord::Migration
+  def change
+    add_column :groups, :role_invoices, :boolean, :default => false, :null => false
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20160217134742) do
+ActiveRecord::Schema.define(version: 20160217194036) do
 
   create_table "article_categories", force: :cascade do |t|
     t.string "name",        limit: 255, default: "", null: false
@@ -140,6 +140,7 @@ ActiveRecord::Schema.define(version: 20160217134742) do
     t.text     "stats",                    limit: 65535
     t.integer  "next_weekly_tasks_number", limit: 4,                              default: 8
     t.boolean  "ignore_apple_restriction",                                        default: false
+    t.boolean  "role_invoices",                                                   default: false, null: false
   end
 
   add_index "groups", ["name"], name: "index_groups_on_name", unique: true, using: :btree