Manually pass owner to initialize Experiment object

rather than do a lookup in the experiments file which is much more
expensive
Provide a re-signup link when deleting a participant

This link can be given to a participant so that they can sign up for a
new slot without having to enter their details again. It also removes
any cookie that’s been set that could potentially stop the participant
signing up.
Manually pass owner to initialize Experiment object

since only the owner has permission to delete, the owner can be passed
as the identity of the logged in user making it impossible for a user
with shared access to delete an experiment of another user.
Unset experiment object before deleting it

to ensure any file handles are closed before removing the file
Return empty array if no exclusions have been set
Manually pass owner as username for own experiments

rather than look up the owner in the experiments file which is more
expensive. In other words, only perform lookup up for shared
experiments.