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

app/models/transfer.rb

@@ -0,0 +1,57 @@
+class Transfer < ApplicationRecord
+  has_many :transactions, dependent: :nullify
+
+  validate :transaction_count, :from_different_accounts, :net_zero_flows, :all_transactions_marked
+
+  def inflow_transaction
+    transactions.find { |t| t.inflow? }
+  end
+
+  def outflow_transaction
+    transactions.find { |t| t.outflow? }
+  end
+
+  def destroy_and_remove_marks!
+    transaction do
+      transactions.each do |t|
+        t.update! marked_as_transfer: false
+      end
+
+      destroy!
+    end
+  end
+
+  class << self
+    def build_from_accounts(from_account, to_account, date:, amount:, currency:, name:)
+      outflow = from_account.transactions.build(amount: amount.abs, currency: currency, date: date, name: name, marked_as_transfer: true)
+      inflow = to_account.transactions.build(amount: -amount.abs, currency: currency, date: date, name: name, marked_as_transfer: true)
+
+      new transactions: [ outflow, inflow ]
+    end
+  end
+
+  private
+
+    def transaction_count
+      unless transactions.size == 2
+        errors.add :transactions, "must have exactly 2 transactions"
+      end
+    end
+
+    def from_different_accounts
+      accounts = transactions.map(&:account_id).uniq
+      errors.add :transactions, "must be from different accounts" if accounts.size < transactions.size
+    end
+
+    def net_zero_flows
+      unless transactions.sum(&:amount).zero?
+        errors.add :transactions, "must have an inflow and outflow that net to zero"
+      end
+    end
+
+    def all_transactions_marked
+      unless transactions.all?(&:marked_as_transfer)
+        errors.add :transactions, "must be marked as transfer"
+      end
+    end
+end