Allow guests to register and call /events?room_id=

This follows the same flows-based flow as regular registration, but as
the only implemented flow has no requirements, it auto-succeeds. In the
future, other flows (e.g. captcha) may be required, so clients should
treat this like the regular registration flow choices.

synapse/api/errors.py

@@ -33,6 +33,7 @@ class Codes(object):
     NOT_FOUND = "M_NOT_FOUND"
     MISSING_TOKEN = "M_MISSING_TOKEN"
     UNKNOWN_TOKEN = "M_UNKNOWN_TOKEN"
+    GUEST_ACCESS_FORBIDDEN = "M_GUEST_ACCESS_FORBIDDEN"
     LIMIT_EXCEEDED = "M_LIMIT_EXCEEDED"
     CAPTCHA_NEEDED = "M_CAPTCHA_NEEDED"
     CAPTCHA_INVALID = "M_CAPTCHA_INVALID"