Refine, add exercises.
Update post 4 to reflect the changes of previous commit.
Fix wtf datetime.
Unify CSS and post 2.
Post 2 exercises.
Tweak post 2, CSS.
Exercises for lesson 3.
Start exercises for 4.
4: delete the h2 part, minor updates.
Style changes etc.
Revert "Temporarily revert "Serve public files as well.""

This reverts commit c67cf108d7c35802f1ec615fb64bce5bff1c3a2f.
Nicer styles for <h1> and <time>