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/models/import/field_test.rb

@@ -0,0 +1,28 @@
+require "test_helper"
+
+class Import::FieldTest < ActiveSupport::TestCase
+  test "key is always a string" do
+    field1 = Import::Field.new label: "Test", key: "test"
+    field2 = Import::Field.new label: "Test2", key: :test2
+
+    assert_equal "test", field1.key
+    assert_equal "test2", field2.key
+  end
+
+  test "can set and override a validator for a field" do
+    field = Import::Field.new \
+      label: "Test",
+      key: "Test",
+      validator: ->(val) { val == 42 }
+
+    assert field.validate(42)
+    assert_not field.validate(41)
+
+    field.define_validator do |value|
+      value == 100
+    end
+
+    assert field.validate(100)
+    assert_not field.validate(42)
+  end
+end