Special case /register for application services

*This issue was originally created by [**@babolivier**](https://github.com/babolivier) at .*

- [ ] Allow an application service to register usernames starting with underscores if its configuration file allows it.
- [ ] Support non-exclusive namespaces.

The latter means we shouldn't respond with `Only 1 match allowed`-like errors if a user ID provided during registration from an appservice matches more than one namespace and none of these namespaces is defined as exclusive. This requires checking whether a namespace is exclusive during registration.

Trace the application-service registration handling and the namespace checks described in the issue, starting with the configuration that controls username namespaces. Verify the underscore allowance and how overlapping exclusive and non-exclusive namespaces are evaluated. Done means both checklist items work without incorrectly returning a single-match error.

Written by the indexing model from the issue text.