Fix Chrome extension installation being offered under Edge

Microsoft Edge unfortunately has to spoof various aspects
of the Chrome environment, including having the string
"Chrome" in its UA string and it also exposes a 'window.chrome' object
in the DOM.

Make the check for the Chrome extension stricter by checking
for 'chrome.webstore' instead.

h/static/scripts/ua-detect.js

@@ -4,5 +4,6 @@ module.exports = {
 
   // we test for the existence of window.chrome here because Microsoft Edge
   // (and possibly others) serve
-  isChrome: typeof window.chrome !== 'undefined',
+  isChrome: typeof window.chrome !== 'undefined' &&
+            typeof window.chrome.webstore !== 'undefined',
 };