Bug 1364894 - Make html-loader treat favicons as dependencies too

Previously `html-loader` only parsed `<img src="...">` tags when
looking for assets/dependencies. Now the `<link href="...">` tags
for favicons are processed too, which means `img/tree_open.png`
and friends will be included in the webpack build and not need to
be manually copied into `dist/img/`:
https://webpack.js.org/loaders/html-loader/

This does not visible change the number of hashed images output to
`dist/`, since the favicons are small enough that `url-loader` inlines
them in the HTML as base64 encoded data URIs (this is adjustable if
not desired later):
https://webpack.js.org/loaders/url-loader/

neutrino-custom/base.js

@@ -104,7 +104,11 @@ module.exports = neutrino => {
     neutrino.config
         .module
         .rule('html')
-        .loader('html', require.resolve('html-loader'));
+        .loader('html', require.resolve('html-loader'), {
+            // Override html-loader's default of `img:src`,
+            // so it also parses favicon images (`<link href="...">`).
+            attrs: ['img:src', 'link:href']
+        });
 
     // Backport Neutrino 8's `test` regex, since Neutrino 4 omitted `.gif`:
     // https://github.com/mozilla-neutrino/neutrino-dev/blob/v4.2.0/packages/neutrino-preset-web/src/index.js#L108