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/models/transfer_test.rb

@@ -0,0 +1,49 @@
+require "test_helper"
+
+class TransferTest < ActiveSupport::TestCase
+  setup do
+    # Transfers can be posted on different dates
+    @outflow = accounts(:checking).transactions.create! date: 1.day.ago.to_date, name: "Transfer to Savings", amount: 100, marked_as_transfer: true
+    @inflow = accounts(:savings).transactions.create! date: Date.current, name: "Transfer from Savings", amount: -100, marked_as_transfer: true
+  end
+
+  test "transfer valid if it has inflow and outflow from different accounts for the same amount" do
+    transfer = Transfer.create! transactions: [ @inflow, @outflow ]
+
+    assert transfer.valid?
+  end
+
+  test "transfer must have 2 transactions" do
+    invalid_transfer_1 = Transfer.new transactions: [ @outflow ]
+    invalid_transfer_2 = Transfer.new transactions: [ @inflow, @outflow, transactions(:savings_four) ]
+
+    assert invalid_transfer_1.invalid?
+    assert invalid_transfer_2.invalid?
+  end
+
+  test "transfer cannot have 2 transactions from the same account" do
+    account = accounts(:checking)
+    inflow = account.transactions.create! date: Date.current, name: "Inflow", amount: -100
+    outflow = account.transactions.create! date: Date.current, name: "Outflow", amount: 100
+
+    assert_raise ActiveRecord::RecordInvalid do
+      Transfer.create! transactions: [ inflow, outflow ]
+    end
+  end
+
+  test "all transfer transactions must be marked as transfers" do
+    @inflow.update! marked_as_transfer: false
+
+    assert_raise ActiveRecord::RecordInvalid do
+      Transfer.create! transactions: [ @inflow, @outflow ]
+    end
+  end
+
+  test "transfer transactions must net to zero" do
+    @outflow.update! amount: 105
+
+    assert_raises ActiveRecord::RecordInvalid do
+      Transfer.create! transactions: [ @inflow, @outflow ]
+    end
+  end
+end