Deletion of objects by the user

Users need to be able to delete (private) objects using the web interface. So far, this is only possible through the command-line. The following objects should be deletable:

• experiments, if not certified (public or private, w/o distinction)
• toolchains, if not used on any experiment. If so, then the experiments must be deleted before the toolchain becomes deletable. I'm not sure what to do with the forking status of forked toolchains.
• algorithms, if not used on any experiment. If so, follow the same rules as for toolchains.
• dataformats, the same as for toolchains and algorithms