Add option to allow registrations that begin with '_'

[hex5f]

This commit resolves a long-standing question dating back to Jira.

leonerd: Should this be a configuration option? We don't want to allow
new user signups beginning with _ on matrix.org, because of clashes
with ASes; but maybe other sites wouldn't mind that?

As this may result in clashes with AppService namespaces, it is disallowed by default. Homeservers which provision users from an external identity provider may find this useful.

Fixes: SYN-738

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