Implement transaction category management (#688)

* Singularize "transaction" in transaction-nested paths

* Refactor category badge partial

* Let modal content define its width

* Add contectual menu to transactions index

* Add null_category helper

* Implement category edits

* Fix inline transaction category badges

* Fix typos in system test paths

* Add missing translations

* Add decoration to color select controller

* Wire up transaction category creation

* Fix indent in color-select-controller

* Add button for clearing category from transaction

* Implement category deletions

* Fix existing modal sizes

* Use null_category in a single place

* Remove anemic method in category deletion controller

* reassign_and_destroy -> reassign_transactions_then_destroy

* Fix i18n

* Remove destroy action from CategoriesController callbacks

* transactions_merchant -> transaction_merchant

* reassign_transactions_then_destroy -> replace_and_destroy

* Add transaction category CRUD tests

* Add presence check for transaction_id

* Check replacement_category_id presence

* Test Transaction::Category#replace_and_destroy!

app/controllers/transactions/categories/deletions_controller.rb

@@ -0,0 +1,24 @@
+class Transactions::Categories::DeletionsController < ApplicationController
+  before_action :set_category
+  before_action :set_replacement_category, only: :create
+
+  def new
+  end
+
+  def create
+    @category.replace_and_destroy! @replacement_category
+
+    redirect_back_or_to transactions_path, notice: t(".success")
+  end
+
+  private
+    def set_category
+      @category = Current.family.transaction_categories.find(params[:transaction_category_id])
+    end
+
+    def set_replacement_category
+      if params[:replacement_category_id].present?
+        @replacement_category = Current.family.transaction_categories.find(params[:replacement_category_id])
+      end
+    end
+end