Make adding push rules idempotent
Make adding push rules idempotent

Also remove the **kwargs from the add_push_rule method.

Fixes https://matrix.org/jira/browse/SYN-391

matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,TribeMedia/synapse,TribeMedia/synapse,TribeMedia/synapse,TribeMedia/synapse,matrix-org/synapse,matrix-org/synapse,TribeMedia/synapse