Added option for a redirect request for oauth login

[jonah-iden]

When logging in via a oauth provider you can now add redirect=someurl in the url query to request to be redirected to some specific site instead of the default configured login success redirect url.
The redirect url has to be in the comma seperated list of whitlisted urls set through oct-redirect-uri-whitelist

How to test

1. create a github oauth provider
2. configure the oct server with the provider
3. create the OCT_REDIRECT_URI_WHITELIST env-variable and add your uri
4. start the server and open http://localhost:8100/api/login/github?token=sometoken&redirect=yourRedirectUri
5. after login is successfull you should be redirected to the given uri.
6. if the uri is not in the whitelist you should get an error

[spoenemann]

Great! Just a question and a request