CSV Transaction Imports (#708)

Introduces a basic CSV import module for bulk-importing account transactions.

Changes include:

- User can load a CSV
- User can configure the column mappings for a CSV
- Imported CSV shows invalid cells
- User can clean up their data directly in the UI
- User can see a preview of the import rows and confirm import
- Layout refactor + Import nav stepper
- System test stability improvements

app/controllers/settings/preferences_controller.rb

@@ -1,4 +1,4 @@
-class Settings::PreferencesController < ApplicationController
+class Settings::PreferencesController < SettingsController
   def edit
   end
 
@@ -14,11 +14,12 @@ class Settings::PreferencesController < ApplicationController
       redirect_to settings_preferences_path, notice: t(".success")
     else
       redirect_to settings_preferences_path, notice: t(".success")
-      render :edit, status: :unprocessable_entity
+      render :show, status: :unprocessable_entity
     end
   end
 
   private
+
     def preference_params
       params.require(:user).permit(family_attributes: [ :id, :currency ])
     end