Support JWT return_to query parameter

Zendesk's JWT authentication method provides a return_to parameter in the initial query payload which can be used to redirect users back to the location they came from. Unfortunately, the Zendesk module currently discards these parameters during the Drupal login redirection chain and the assembly of the JWT authentication payload.

A patch is attached which:

1. Encodes the return_to parameter into the destination= param for the Drupal-side user-login redirect and
2. Adds this query parameter to the JWT authentication payload so that users end up at the right location in Zendesk.