Add option to related docs items to reorder them

When related doc links are added to a document, the only option that a user has is to remove an item. Other multiple, reorderable fields have these options. Not so long ago this was added to the taxonomy plugin. It would be a nice and easy addition to the related docs as well.

Layout for the control is in this file: hippo-plugin-relateddocs/addon-cms/src/main/java/org/onehippo/forge/relateddocs/editor/RelatedDocsPlugin.html