Allow deployer to set a migration-done flag

* Save deployer's address on contract initialization
* Add modifier for requiring tx sender to be deployer
* Add migrationDone flag and function to set it to finished status

hardhat.config.js

@@ -1,6 +1,7 @@
 require("@nomiclabs/hardhat-waffle");
 require("hardhat-deploy");
 require("hardhat-deploy-ethers");
+require("@nomicfoundation/hardhat-chai-matchers");
 require("@openzeppelin/hardhat-upgrades");
 const Kredits = require("./lib/kredits");