"format=mutable" in the web API

[zooko]

According to webapi.rst​, you have to specify either format=MDMF or format=SDMF when creating a mutable file. However the user of an API might not know or care what is the difference between those two, and might also want to automatically start using a newer format if that newer format becomes the default, without changing the client code. This ticket is to add format=mutable which uses the default mutable format.