Provide a way to delete aliases

[pilif]

While the server already creates and stores an update key with each alias, this is currently not used. The REST-API doesn't expose any way of deleting an alias once it has been created. This is not so bad as it's easy to just not use an erroneously generated alias, but still, it might be of use.

To delete an alias,

DELETE /aliases/<id>

and in the body

{updatekey: <the key you got when you created the alias>}

or DELETE /aliases/<id>?updatekey=...

I'm undecided. Having a request body for DELETE seems unclean, but the key isn't quite a query parameter either.