Fix Hammer.JS error on web pages that use Require.JS

Fix an error related to Hammer.JS on client startup on pages that define
a global variable called `define`, which is part of Require.JS. Hammer's UMD
bundle does not set `module.exports` in this case, so modules that import it
get an empty object back.

Fix the issue by making sure that any reference to `define` inside
bundles evaluates to undefined (assuming there is no local variable
within the module with that name).

This fixes an issue where clicking "Annotate" failed to open the sidebar
on BBC News articles (eg. https://www.bbc.co.uk/news/world-52144390).

scripts/gulp/create-bundle.js

@@ -87,11 +87,19 @@ module.exports = function createBundle(config, buildOpts) {
     // modules provide the implementations of these.
     builtins: ['console', '_process', 'querystring'],
     externalRequireName,
+
+    // Map of global variable names to functions returning _source code_ for
+    // the values of those globals.
     insertGlobalVars: {
+      // Workaround for Hammer.JS on pages that use RequireJS. Hammer doesn't
+      // set `module.exports` if a global variable called `define` exists.
+      define: () => 'undefined',
+
       // The Browserify polyfill for the `Buffer` global is large and
       // unnecessary, but can get pulled into the bundle by modules that can
       // optionally use it if present.
       Buffer: undefined,
+
       // Override the default stub for the `global` var which defaults to
       // the `global`, `self` and `window` globals in that order.
       //