added a command to the admin bot to create a new user, even with registration disabled

I've added a command to the admin bot to create a new user, even with registration disabled. Besides running cargo test I also did this:

1. Run conduit env CONDUIT_CONFIG=./tests/test-config.toml cargo run --release
2. run element web instance with docker docker run -p 80:80 vectorim/element-web
3. open localhost:80 and interact with server on http://localhost:6167
4. create an admin user the normal way
5. in admin chat I wrote @conduit:localhost: create-user tony
6. logged in as user and sent message to admin
7. checked I received the message

But I'm not very familiar with conduit code yet so please independently test and verify before approving anything.

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• I ran cargo fmt and cargo test
• I agree to release my code and all other changes of this MR under the Apache-2.0 license