Prevent caching of the entry point for QA deployments

The short-lived max-age value set previously was overridden by the
higher value of 1800 (30 mins) set in CloudFlare's "Browser Cache TTL"
setting under Page Rules for cdn.hypothes.is.

Resolve this by simply disabling caching of the entry point for qa
deployments. This means that https://cdn.hypothes.is/hypothesis@qa will
always return the boot script for the latest client QA release.

Jenkinsfile

@@ -104,7 +104,7 @@ node {
                 sh """
                 export AWS_ACCESS_KEY_ID=${env.AWS_ACCESS_KEY_ID}
                 export AWS_SECRET_ACCESS_KEY=${env.AWS_SECRET_ACCESS_KEY}
-                scripts/deploy-to-s3.js --bucket ${s3Bucket} --tag qa
+                scripts/deploy-to-s3.js --bucket ${s3Bucket} --tag qa --no-cache-entry
                 """
             }
         }

scripts/deploy-to-s3.js

@@ -84,7 +84,7 @@ class S3Uploader {
  * are set to be cached indefinitely by the browser. Keys that are pointers
  * to the current version only have a short cache lifetime.
  */
-async function uploadPackageToS3(bucket, tag='') {
+async function uploadPackageToS3(bucket, options) {
   // Get list of files that are distributed with the package, respecting
   // the `files` field in `package.json`, `.npmignore` etc.
   const files = await packlist({ path: '.' });
@@ -119,23 +119,39 @@ async function uploadPackageToS3(bucket, tag='') {
   // Upload a copy of the entry-point to `$PACKAGE_NAME` or `$PACKAGE_NAME@$TAG`.
   // This enables creating URLs that always point to the current version of
   // the package.
+  let aliasCacheControl;
+  if (options.cacheEntry) {
+    // nb. When deploying to cdn.hypothes.is, the max-age seen by the browser is
+    // the higher of the value here and CloudFlare's "Browser Cache TTL"
+    // setting.
+    aliasCacheControl = 'public, max-age=1800, must-revalidate';
+  } else {
+    aliasCacheControl = 'no-cache';
+  }
+
   let aliasPath;
-  if (!tag) {
+  if (!options.tag) {
     aliasPath = `${packageName}`;
   } else {
-    aliasPath = `${packageName}@${tag}`;
+    aliasPath = `${packageName}@${options.tag}`;
   }
   await uploader.upload(aliasPath, entryPoint, {
-    cacheControl:  'public, max-age=30, must-revalidate',
+    cacheControl: aliasCacheControl,
   });
 }
 
 commander
   .option('--bucket [bucket]', 'S3 bucket name')
   .option('--tag [tag]', 'Version tag')
+  .option('--no-cache-entry', 'Prevent CDN/browser caching of entry point')
   .parse(process.argv);
 
-uploadPackageToS3(commander.bucket, commander.tag)
+const options = {
+  tag: commander.tag,
+  cacheEntry: commander.cacheEntry,
+};
+
+uploadPackageToS3(commander.bucket, options)
   .catch(err => {
     console.error('Failed to upload S3 package', err);
     process.exit(1);