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/controllers/transfers_controller.rb

@@ -0,0 +1,41 @@
+class TransfersController < ApplicationController
+  layout "with_sidebar"
+
+  before_action :set_transfer, only: :destroy
+
+  def new
+    @transfer = Transfer.new
+  end
+
+  def create
+    from_account = Current.family.accounts.find(transfer_params[:from_account_id])
+    to_account = Current.family.accounts.find(transfer_params[:to_account_id])
+
+    @transfer = Transfer.build_from_accounts from_account, to_account, \
+                                             date: transfer_params[:date],
+                                             amount: transfer_params[:amount].to_d,
+                                             currency: transfer_params[:currency],
+                                             name: transfer_params[:name]
+
+    if @transfer.save
+      redirect_to transactions_path, notice: t(".success")
+    else
+      render :new, status: :unprocessable_entity
+    end
+  end
+
+  def destroy
+    @transfer.destroy_and_remove_marks!
+    redirect_back_or_to transactions_url, notice: t(".success")
+  end
+
+  private
+
+    def set_transfer
+      @transfer = Transfer.find(params[:id])
+    end
+
+    def transfer_params
+      params.require(:transfer).permit(:from_account_id, :to_account_id, :amount, :currency, :date, :name)
+    end
+end