Send a permanent link for user invitation in user registration addon

• Install user registration addon
• Invite a user to access a space
• Approve request as an admin

=> In the invitation email sent, the link to the space user is invited to looks like:

Once your invitation is validated, you'll be able to log in to the application and access <a href="http://localhost:8080/nuxeo/nxpath/default/default-domain/workspaces/toto@view_documents?conversationId=0NXMAIN1">toto</a>.

=> link should be the permanent link to the document instead.