Zendesk sso 2

Add the dashboard-side support for zendesk sso (agents and end users log in with their dashboard/learn.code.org accounts -- agents and admins must log in, users may contact us and browse forums/questions without logging in, users must log in to post forums/questions).

this might need to be a string constant, it seems JWT expects a non-nil string as its secret param (for when the CI's deploy variables aren't connected to dashboard, e.g. when running tests locally)