mod_http_oauth2: Add role selector to consent page List includes all roles available to the user, if more than one. Defaults to either the first role in the scope string or the users primary role. Earlier draft listed all roles, but having options that can't be selected is bad UX and the entire list of all roles on the server could be long, and perhaps even sensitive. Allows e.g. picking a role with fewer permissions than what might otherwise have been selected. UX wise, doing this with more checkboxes or possibly radio buttons would have been confusion and/or looked messier. Fixes the previous situation where unselecting a role would default to the primary role, which could be more permissions than requested.