Use identicons as fallback for libravatar

The usage of identicons makes users more distinguishable as when only the default librvatar image is used.
This only applies to users that have no avatar on libravatar or gravatar.

Signed-off-by: Erik Michelson <github@erik.michelson.eu>

lib/letter-avatars.js

@@ -37,11 +37,11 @@ exports.generateAvatarURL = function (name, email = '', big = true) {
   const hexDigest = hash.digest('hex')
 
   if (email !== '' && config.allowGravatar) {
-    photo = 'https://cdn.libravatar.org/avatar/' + hexDigest
+    photo = `https://cdn.libravatar.org/avatar/${hexDigest}?default=identicon`
     if (big) {
-      photo += '?s=400'
+      photo += '&s=400'
     } else {
-      photo += '?s=96'
+      photo += '&s=96'
     }
   } else {
     photo = config.serverURL + '/user/' + (name || email.substring(0, email.lastIndexOf('@')) || hexDigest) + '/avatar.svg'