"format=mutable" in the web API

[zooko]

According to [trunk/docs/frontends/webapi.rst?rev=0ef593947755a8b81edc73d033219724268faf80#writing-uploading-a-file], 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.