[task] Move "/callback" path from client route to server route

[ivicac]

• To use ConnectPortal dialog in the embedded mode, OAuth callback has to call a server-side based URL instead of the one defined on the client side.
  -The server side should, after saving incoming data, redirect to the client.

[ivicac]

It seems it is not possible right now because we still need to save other connection data inserted in ConnectionDialog, together with oauth token