"push" bot user to matrix-server at start

[olmari]

AFAIK currently appservice-slack "says" pretty much zero things or events towards matrix-server when (first) booted (at least I didn't catch any on synapse logs, but this is not definitive), so matrix-server can't know about bot user exist.. Thus for example the when inviting bot user to admin channel matrix-client will say bot user does not exist.

bridge should at least make the user known to matrix-server so it wouldn't complain about invitation...

and that would also make debugging (new) config easier as bridge-service would say something to matrix-server and that would show on logs, and also matrix-client not complain about bot not existing and just invite it.