stick sidebar footer with a flex sidebar

[eine]

This PR makes toc-wrapper a flex column, which allows #toc to grow. As a result, toc-footer blocks are sticked to the bottom of the visible area (size of the window).

Unfortunately, the modification of margin-top is ignored.

It can be forced with !important, but I'd prefer to understand what's going on. I could not find out in which other source is it overwritten. It seems not to be any of the locally available sources (which I may modify).