Add t=mkdir-immutable to the webapi. Closes #607.

Add t=mkdir-immutable to the webapi. Closes #607.

* change t=mkdir-with-children to not use multipart/form encoding. Instead,
  the request body is all JSON. t=mkdir-immutable uses this format too.
* make nodemaker.create_immutable_dirnode() get convergence from SecretHolder,
  but let callers override it
* raise NotDeepImmutableError instead of using assert()
* add mutable= argument to DirectoryNode.create_subdirectory(), default True