ci - announce QA build to GitHub pull request

.circleci/config.yml

@@ -100,6 +100,18 @@ jobs:
             - dist
       - store_artifacts:
           path: dist/mascara
+      - run:
+          name: build:announce
+          command: |
+            CIRCLE_PR_NUMBER="${CIRCLE_PR_NUMBER:-${CIRCLE_PULL_REQUEST##*/}}"
+            SHORT_SHA1=$(echo $CIRCLE_SHA1 | cut -c 1-7)
+            QA_BUILD_URL="https://$CIRCLE_BUILD_NUM-42009758-gh.circle-artifacts.com/0/home/circleci/project/dist/mascara/home.html"
+            COMMENT_BODY="[Try QA Build for $SHORT_SHA1]($QA_BUILD_URL)"
+            JSON_PAYLOAD="{\"body\":\"$COMMENT_BODY\"}"
+            POST_COMMENT_URI="https://api.github.com/repos/metamask/metamask-extension/issues/$CIRCLE_PR_NUMBER/comments"
+            echo "Announcement:\n$COMMENT_BODY"
+            echo "Posting to $POST_COMMENT_URI"
+            curl -d "$JSON_PAYLOAD" -H "Authorization: token $GITHUB_COMMENT_TOKEN" $POST_COMMENT_URI
 
   prep-scss:
     docker: