Allow setting any contributor accounts

This allows to pass in an account object when creating or updating a
contribtor.

lib/serializers/contributor.js

@@ -28,6 +28,7 @@ class Contributor {
       github_username,
       gitea_username,
       wiki_username,
+      accounts
     } = this;
 
     let data = {
@@ -35,7 +36,7 @@ class Contributor {
       '@type': 'Contributor',
       kind,
       name,
-      'accounts': [],
+      'accounts': (accounts || []),
     };
 
     if (url) {