Merge branch 'manifest' into 'develop'

Link to manifest.json

Closes #412

See merge request soapbox-pub/soapbox-fe!891

app/index.ejs

@@ -5,6 +5,7 @@
     <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover, user-scalable=no">
     <meta name="mobile-web-app-capable" content="yes">
     <meta name="apple-mobile-web-app-capable" content="yes">
+    <link href="/manifest.json" rel="manifest">
     <!--server-generated-meta-->
     <link rel="icon" type="image/png" href="/favicon.png">
   </head>