Fix warnings about not calling superclass constructor
Fix warnings about not calling superclass constructor

Separate `SimpleCommand` from `Command`, so that things which don't want to use
the `data` property don't have to, and thus fix the warnings PyCharm was giving
me about not calling `__init__` in the base class.

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