Add slug string column to Budget, Budget::Heading and Budget::Group

Why:

* We're still using id's on routes and other parts of the app, using slugs are more friendly and nice

How:

* Just a migration with a slug column with string type

db/migrate/20170704105112_add_slugs_to_budget_heading_group.rb

@@ -0,0 +1,7 @@
+class AddSlugsToBudgetHeadingGroup < ActiveRecord::Migration
+  def change
+    add_column :budgets, :slug, :string
+    add_column :budget_groups, :slug, :string
+    add_column :budget_headings, :slug, :string
+  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: 20170703120055) do
+ActiveRecord::Schema.define(version: 20170704105112) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -103,6 +103,7 @@ ActiveRecord::Schema.define(version: 20170703120055) do
   create_table "budget_groups", force: :cascade do |t|
     t.integer "budget_id"
     t.string  "name",      limit: 50
+    t.string  "slug"
   end
 
   add_index "budget_groups", ["budget_id"], name: "index_budget_groups_on_budget_id", using: :btree
@@ -112,6 +113,7 @@ ActiveRecord::Schema.define(version: 20170703120055) do
     t.string  "name",       limit: 50
     t.integer "price",      limit: 8
     t.integer "population"
+    t.string  "slug"
   end
 
   add_index "budget_headings", ["group_id"], name: "index_budget_headings_on_group_id", using: :btree
@@ -196,6 +198,7 @@ ActiveRecord::Schema.define(version: 20170703120055) do
     t.text     "description_balloting"
     t.text     "description_reviewing_ballots"
     t.text     "description_finished"
+    t.string   "slug"
   end
 
   create_table "campaigns", force: :cascade do |t|