Add CSRF tokens to the API

[Senyoret1]

Feature description A new API endpoint for creating CSRF tokens should be added and all potentially dangerous API endpoints should request the token. In the desktop wallet it is implemented in https://github.com/skycoin/skycoin/blob/develop/src/api/README.md#csrf

In the desktop wallet the token is requested for POST request, but if there are GET request potentially needing it, it should be requested for those request too.