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/models/import/field.rb

@@ -0,0 +1,32 @@
+class Import::Field
+  def self.iso_date_validator(value)
+    Date.iso8601(value)
+    true
+  rescue
+    false
+  end
+
+  def self.bigdecimal_validator(value)
+    BigDecimal(value)
+    true
+  rescue
+    false
+  end
+
+  attr_reader :key, :label, :validator
+
+  def initialize(key:, label:, validator: nil)
+    @key = key.to_s
+    @label = label
+    @validator = validator
+  end
+
+  def define_validator(validator = nil, &block)
+    @validator = validator || block
+  end
+
+  def validate(value)
+    return true if validator.nil?
+    validator.call(value)
+  end
+end