Add stub service worker so users can install on desktop with Chrome

Signed-off-by: Dale Harvey <dale@arandomurl.com>

res/sw.js

@@ -0,0 +1 @@
+self.addEventListener('fetch',() => {});

scripts/copy-res.js

@@ -58,6 +58,7 @@ const INCLUDE_LANGS = [
 // "dest/b/...".
 const COPY_LIST = [
     ["res/manifest.json", "webapp"],
+    ["res/sw.js", "webapp"],
     ["res/welcome.html", "webapp"],
     ["res/welcome/**", "webapp/welcome"],
     ["res/themes/**", "webapp/themes"],

src/vector/index.html

@@ -41,6 +41,11 @@
         window.vector_indexeddb_worker_script = '<%= htmlWebpackPlugin.files.chunks['indexeddb-worker'].entry %>';
     </script>
     <script src="<%= htmlWebpackPlugin.files.chunks['bundle'].entry %>"></script>
+    <script>
+      if ('serviceWorker' in navigator) {
+        navigator.serviceWorker.register('sw.js');
+      }
+    </script>
     <img src="<%= require('matrix-react-sdk/res/img/warning.svg') %>" width="24" height="23" style="visibility: hidden; position: absolute; top: 0px; left: 0px;"/>
     <img src="<%= require('matrix-react-sdk/res/img/e2e/warning.svg') %>" width="24" height="23" style="visibility: hidden; position: absolute; top: 0px; left: 0px;"/>
     <img src="<%= require('matrix-react-sdk/res/img/feather-customised/warning-triangle.svg') %>" width="24" height="23" style="visibility: hidden; position: absolute; top: 0px; left: 0px;"/>