Transaction transfers, payments, and matching (#883)

* Add transfer model and clean up family snapshot fixtures

* Ignore transfers in income and expense snapshots

* Add transfer validations

* Implement basic transfer matching UI

* Fix merge conflicts

* Add missing translations

* Tweak selection states for transfer types

* Add missing i18n translation

test/support/family_snapshot_test_helper.rb

@@ -0,0 +1,21 @@
+module FamilySnapshotTestHelper
+  # See: https://docs.google.com/spreadsheets/d/18LN5N-VLq4b49Mq1fNwF7_eBiHSQB46qQduRtdAEN98/edit?usp=sharing
+  def get_expected_balances_for(key)
+    expected_results_file.map do |row|
+      {
+        date: (Date.current - row["date_offset"].to_i.days).to_date,
+        balance: row[key.to_s].to_d
+      }
+    end
+  end
+
+  def get_today_snapshot_value_for(metric)
+    expected_results_file[-1][metric.to_s].to_d
+  end
+
+  private
+
+    def expected_results_file
+      CSV.read("test/fixtures/files/expected_family_snapshots.csv", headers: true)
+    end
+end