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/transactions/merchants_controller.rb

@@ -1,4 +1,6 @@
 class Transactions::MerchantsController < ApplicationController
+  layout "with_sidebar"
+
   before_action :set_merchant, only: %i[ edit update destroy ]
 
   def index