Re-deploy the Cognito domain with a new name

gh-274

This is to complete the 2-step deploy described here:

aws/aws-cdk#10062 (comment)

I'm renaming the Cognito User Pool Domain with a fixed-size string with
acceptable letters for a Domain prefix, and that varies based on the
repo namespace, repo name, branch, and account.

lib/madliberation-stack.ts

@@ -123,11 +123,11 @@ export class MadliberationStack extends cdk.Stack {
       .slice(0, 20);
     const domainPrefix = stacknameHash + this.account;
 
-    // userPool.addDomain("UserPoolDomain", {
-    //   cognitoDomain: {
-    //     domainPrefix: domainPrefix,
-    //   },
-    // });
+    userPool.addDomain("UserPoolDomain", {
+      cognitoDomain: {
+        domainPrefix: domainPrefix,
+      },
+    });
 
     new cdk.CfnOutput(this, "DistributionDomainName", {
       value: distro.distributionDomainName,