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/jobs/import_job.rb

@@ -0,0 +1,7 @@
+class ImportJob < ApplicationJob
+  queue_as :default
+
+  def perform(import)
+    import.publish
+  end
+end