reserve user.tngl.sh handles to original account when switching to custom domain handles #313

This is something that the main Bluesky Social PDSes do when switching to custom domain handles to prevent impersonation related issues. The previous tngl.sh handle should be only available to the original account unless changed to another tngl.sh handle or deleted permanently.

Note that this should apply to the Tangled PDS (or its self-hosted equivalent for those self-hosting themselves) and other PDSes may do this on their own.