Implement multi-part exercises#402

Draft

YoanwM wants to merge 8 commits intoocaml-sf:masterfrom

pfitaxel:learnocaml_v3

Commits on May 14, 2021

Make the prelude available in description page
Yoan Mollet
committed

Commits on May 18, 2021

Change the +/- opening skills/requirements in the meta (description page)
Yoan Mollet
committed

Commits on Jun 23, 2021

New exercise version
Yoan Mollet
committed
New exercise version 3
Yoan Mollet
committed

Commits on Jul 6, 2021

Merge branch 'learnocaml_v3' of github.com:pfitaxel/learn-ocaml into learnocaml_v3
Yoan Mollet
committed
Update of multi-part exercises data & build (check_all_against)
Yoan Mollet
committed

Commits on Jul 7, 2021

Demo of a multi-part exercise
Yoan Mollet
committed
Make the "AllGrade!" button unavailable in non multi-part exercises
Yoan Mollet
committed