create support user (#4141)

Allow for the creation of a support user.

A support user can access the server, join rooms, interact with other users, but does not appear in the user directory nor does it contribute to monthly active user limits.

synapse/api/auth.py

@@ -802,9 +802,10 @@ class Auth(object):
             threepid should never be set at the same time.
         """
 
-        # Never fail an auth check for the server notices users
+        # Never fail an auth check for the server notices users or support user
         # This can be a problem where event creation is prohibited due to blocking
-        if user_id == self.hs.config.server_notices_mxid:
+        is_support = yield self.store.is_support_user(user_id)
+        if user_id == self.hs.config.server_notices_mxid or is_support:
             return
 
         if self.hs.config.hs_disabled:

synapse/api/constants.py

@@ -119,3 +119,11 @@ KNOWN_ROOM_VERSIONS = {
 
 ServerNoticeMsgType = "m.server_notice"
 ServerNoticeLimitReached = "m.server_notice.usage_limit_reached"
+
+
+class UserTypes(object):
+    """Allows for user type specific behaviour. With the benefit of hindsight
+    'admin' and 'guest' users should also be UserTypes. Normal users are type None
+    """
+    SUPPORT = "support"
+    ALL_USER_TYPES = (SUPPORT)