voiprestservlet is broken

[richvdh]

suspect this was during the kill-off-v2 refactor:

2019-06-03 17:22:23,223 - synapse.http.server - 112 - ERROR - GET-73899- Failed handle request via 'VoipRestServlet': <XForwardedForRequest at 0x7f2d4e05dfd0 method='GET' uri='/_matrix/client/r0/voip/turnServer' clientproto='HTTP/1.1' site=8008>
Traceback (most recent call last):
  File /opt/synapse/synapse/synapse/http/server.py, line 81, in wrapped_request_handler
    yield h(self, request)
  File /opt/synapse/synapse/synapse/http/server.py, line 316, in _async_render
    callback_return = yield callback(request, **kwargs)
  File /opt/synapse/env3/lib/python3.5/site-packages/twisted/internet/defer.py, line 1418, in _inlineCallbacks
    result = g.send(result)
  File /opt/synapse/synapse/synapse/rest/client/v1/voip.py, line 35, in on_GET
    requester = yield self.auth.get_user_by_req(
AttributeError: 'VoipRestServlet' object has no attribute 'auth'

[richvdh]

think this is a regression, so marking as a release-blocker