feat(netlify): inject netlify build environments into static site

This should (hopefully) inject the netlify build environment
variables into the static site, so that we can display more
meaningful information about the deployment.

www/config.js

@@ -13,11 +13,21 @@ const bootstrapVersion = require('bootstrap/package.json').version;
 
 const shortVersion = bootstrapVersion.split('.').slice(0, 2).join('.');
 
+const netlify =
+  process.env.NETLIFY === 'true'
+    ? {
+        pullRequest: process.env.PULL_REQUEST,
+        reviewId: process.env.REVIEW_ID,
+        branch: process.env.BRANCH,
+      }
+    : null;
+
 const config = {
   bootstrapVersion,
   docsUrl: `https://getbootstrap.com/docs/${shortVersion}`,
   version: require('../package.json').version,
   cssHash: getIntegrity('bootstrap/dist/css/bootstrap.min.css'),
+  netlify,
 };
 
 module.exports = config;

www/src/components/MasterDocsAlert.js

@@ -3,25 +3,26 @@ import Alert from 'react-bootstrap/Alert';
 import Container from 'react-bootstrap/Container';
 import Row from 'react-bootstrap/Row';
 
-function MasterDocsAlert() {
+export default function MasterDocsAlert() {
+  const { netlify } = config;
   let deploymentDetails = null;
-  if (process.env.NETLIFY) {
-    if (process.env.PULL_REQUEST === 'true') {
+  if (netlify) {
+    if (netlify.pullRequest) {
       deploymentDetails = (
         <span>
           a{' '}
           <Alert.Link
-            href={`https://github.com/react-bootstrap/react-bootstrap/pull/${process.env.REVIEW_ID}`}
+            href={`https://github.com/react-bootstrap/react-bootstrap/pull/${netlify.reviewId}`}
           >
             pull request
           </Alert.Link>
         </span>
       );
     } else {
-      deploymentDetails = `the ${process.env.BRANCH} branch`;
+      deploymentDetails = `the ${netlify.branch} branch`;
     }
   }
-  return process.env.NETLIFY ? (
+  return netlify ? (
     <Container fluid>
       <Row>
         <Alert variant="warning" className="w-100">
@@ -35,5 +36,3 @@ function MasterDocsAlert() {
     </Container>
   ) : null;
 }
-
-export default MasterDocsAlert;