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

db/migrate/20240502205006_create_imports.rb

@@ -0,0 +1,15 @@
+class CreateImports < ActiveRecord::Migration[7.2]
+  def change
+    create_enum :import_status, %w[pending importing complete failed]
+
+    create_table :imports, id: :uuid do |t|
+      t.references :account, null: false, foreign_key: true, type: :uuid
+      t.jsonb :column_mappings
+      t.enum :status, enum_type: :import_status, default: "pending"
+      t.string :raw_csv_str
+      t.string :normalized_csv_str
+
+      t.timestamps
+    end
+  end
+end