Add ability to enter promotions when creating transaction

Groceries/Transactions/NewTransactionPromotion_Modal.cshtml

@@ -0,0 +1,16 @@
+@using Groceries.Data
+
+@model Transaction
+@{
+    Layout = "_Modal";
+    ViewBag.Title = "New Transaction Promotion";
+}
+
+<form class="card__content" id="newTransactionPromotion" method="post" asp-action="NewTransactionPromotion" data-action="turbo:submit-end->modal#close">
+    <partial name="_TransactionPromotionForm" model="(Model, (TransactionPromotion?)null)" />
+</form>
+
+<footer class="card__footer card__footer--shaded row">
+    <button class="button button--primary" type="submit" form="newTransactionPromotion">Add</button>
+    <button class="button" data-action="modal#close">Cancel</button>
+</footer>