Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/utils/escrow/ConditionalEscrow.sol

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+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts v4.4.1 (utils/escrow/ConditionalEscrow.sol)
+
+pragma solidity ^0.8.0;
+
+import "./Escrow.sol";
+
+/**
+ * @title ConditionalEscrow
+ * @dev Base abstract escrow to only allow withdrawal if a condition is met.
+ * @dev Intended usage: See {Escrow}. Same usage guidelines apply here.
+ */
+abstract contract ConditionalEscrow is Escrow {
+    /**
+     * @dev Returns whether an address is allowed to withdraw their funds. To be
+     * implemented by derived contracts.
+     * @param payee The destination address of the funds.
+     */
+    function withdrawalAllowed(address payee) public view virtual returns (bool);
+
+    function withdraw(address payable payee) public virtual override {
+        require(withdrawalAllowed(payee), "ConditionalEscrow: payee is not allowed to withdraw");
+        super.withdraw(payee);
+    }
+}