[backend] Refactor FB & Google login flow to prevent the jwt token to be sent as a query parameter

[fernanri]

The current FB login flow is being made in the backend side and the resultant jwt token is then being transmitted to the frontend as a query parameter in a redirect call. This makes the jwt token to be leaked in the server logs. (at least) which is not optimal.

The proposal is to perform the full oAuth authentication flow, where the client (frontend) asks for the FB token and then calls an API method with that token. The API should then use the token to fetch the information needed and return a JWT token in the reply.