Redirect to external sites with temporary token

[Max-Levitskiy]

We shouldn't use same token for all sites from security reasons. Also, we want to avoid stilling and using on redirect with a token in GET request param.
We need to create a temporary token and redirect with it. From this temp token, we need to make a request to auth to get a real token for a union. The request should be signed by the union's app id and secret. From auth backend side we check if the token is correct and generate a new token for the union.
For the implementation of this functionality, we need to add information about union token was generated for in token.