Move old `exercises/concept.wip` drafts

[colinleach]

These older drafts are causing confusion as we try to get a new learning syllabus together. They are hidden from students, but show up to maintainers in distracting and often puzzling ways.

Proposals:

• Delete the duplicate leap exercise completely (as already discussed with @depial in #784).
• Move the others to somewhere they will be ignored by the website: out of the exercises directory, to some neutral name like drafts.
• Re-use the exercise runtests.jl and example.jl as appropriate, but expect to rewrite the markdown files pretty much completely, based on reviewer feedback.
• With a bit less urgency, also move the concepts.wip directory from the top level (note the plural concepts in this case)