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

test/system/settings_test.rb

@@ -14,6 +14,7 @@ class SettingsTest < ApplicationSystemTestCase
       [ "Categories", "Categories", transaction_categories_path ],
       [ "Merchants", "Merchants", transaction_merchants_path ],
       [ "Rules", "Rules", transaction_rules_path ],
+      [ "Imports", "Imports", imports_path ],
       [ "What's New", "What's New", changelog_path ],
       [ "Feedback", "Feedback", feedback_path ],
       [ "Invite friends", "Invite friends", invites_path ]
@@ -27,6 +28,7 @@ class SettingsTest < ApplicationSystemTestCase
   end
 
   private
+
     def open_settings_from_sidebar
       find("#user-menu").click
       click_link "Settings"