Issues/222 category editing add order interface

Builds on https://github.com/mysociety/alaveteli/pull/1721 to add an admin interface for reordering headings and categories. 
This line is causing a migration failure as it already exists https://github.com/mysociety/alaveteli/commit/6186bc07c36abb1ebe153fb4eb9967d8ece5e414#diff-f19615febb3590dc659f769ce1c8c976R6

```sh
vagrant@vagrant-ubuntu-precise-64:~/alaveteli$ bundle exec rake db:migrate
==  CreatePublicBodyHeadingsAndCategories: migrating ==========================
-- create_table(:public_body_headings, {:force=>true})
NOTICE:  CREATE TABLE will create implicit sequence "public_body_headings_id_seq" for serial column "public_body_headings.id"
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "public_body_headings_pkey" for table "public_body_headings"
   -> 0.0069s
-- create_table(:public_body_categories, {:force=>true})
NOTICE:  CREATE TABLE will create implicit sequence "public_body_categories_id_seq" for serial column "public_body_categories.id"
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "public_body_categories_pkey" for table "public_body_categories"
   -> 0.0231s
-- create_table(:public_body_categories_public_body_headings, {:id=>false})
   -> 0.0011s
==  CreatePublicBodyHeadingsAndCategories: migrated (0.0340s) =================

==  CreateCategoryTranslationTables: migrating ================================
NOTICE:  CREATE TABLE will create implicit sequence "public_body_category_translations_id_seq" for serial column "public_body_category_translations.id"
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "public_body_category_translations_pkey" for table "public_body_category_translations"
NOTICE:  CREATE TABLE will create implicit sequence "public_body_heading_translations_id_seq" for serial column "public_body_heading_translations.id"
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "public_body_heading_translations_pkey" for table "public_body_heading_translations"
-- remove_column(:public_body_headings, :locale)
   -> 0.0010s
-- remove_column(:public_body_categories, :locale)
   -> 0.0008s
-- add_index(:public_body_categories, :category_tag, {:unique=>true})
   -> 0.0033s
==  CreateCategoryTranslationTables: migrated (0.0963s) =======================

==  AddDisplayOrderToCategoriesAndHeadings: migrating =========================
-- add_column(:public_body_categories_public_body_headings, :category_display_order, :integer)
   -> 0.0010s
-- add_column(:public_body_headings, :display_order, :integer)
rake aborted!
An error has occurred, this and all later migrations canceled:

PG::Error: ERROR:  column "display_order" of relation "public_body_headings" already exists
: ALTER TABLE "public_body_headings" ADD COLUMN "display_order" integer

Tasks: TOP => db:migrate
(See full trace by running task with --trace)
```