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/csv.rb

@@ -0,0 +1,74 @@
+class Import::Csv
+  def self.parse_csv(csv_str)
+    CSV.parse((csv_str || "").strip, headers: true, converters: [ ->(str) { str.strip } ])
+  end
+
+  def self.create_with_field_mappings(raw_csv_str, fields, field_mappings)
+    raw_csv = self.parse_csv(raw_csv_str)
+
+    generated_csv_str = CSV.generate headers: fields.map { |f| f.key }, write_headers: true do |csv|
+      raw_csv.each do |row|
+        row_values = []
+
+        fields.each do |field|
+          # Finds the column header name the user has designated for the expected field
+          mapped_field_key = field_mappings[field.key] if field_mappings
+          mapped_header = mapped_field_key || field.key
+
+          row_values << row.fetch(mapped_header, "")
+        end
+
+        csv << row_values
+      end
+    end
+
+    new(generated_csv_str)
+  end
+
+  attr_reader :csv_str
+
+  def initialize(csv_str, column_validators: nil)
+    @csv_str = csv_str
+    @column_validators = column_validators || {}
+  end
+
+  def table
+    @table ||= self.class.parse_csv(csv_str)
+  end
+
+  def update_cell(row_idx, col_idx, value)
+    copy = table.by_col_or_row
+    copy[row_idx][col_idx] = value
+    copy
+  end
+
+  def valid?
+    table.each_with_index.all? do |row, row_idx|
+      row.each_with_index.all? do |cell, col_idx|
+        cell_valid?(row_idx, col_idx)
+      end
+    end
+  end
+
+  def cell_valid?(row_idx, col_idx)
+    value = table.dig(row_idx, col_idx)
+    header = table.headers[col_idx]
+    validator = get_validator_by_header(header)
+    validator.call(value)
+  end
+
+  def define_validator(header_key, validator = nil, &block)
+    header = table.headers.find { |h| h.strip == header_key }
+    raise "Cannot define validator for header #{header_key}: header does not exist in CSV" if header.nil?
+
+    column_validators[header] = validator || block
+  end
+
+  private
+
+    attr_accessor :column_validators
+
+    def get_validator_by_header(header)
+      column_validators&.dig(header) || ->(_v) { true }
+    end
+end