Session cover pages

[encukou]

It would be great to have “cover pages” for sessions (sets of lectures that are done in one day on a run).
There should be a “front page” at /<year>/<run>/sessions/<session>/front/ containing:

• what we'll learn today
• initial instructions for students
• link to notes for coaches
  • [content] For Asteroids, add reference code here (from Add reference code for Asteroids naucse.python.cz#129)
• link to feedback (added after the session ends)

and a “back page” at /<year>/<run>/sessions/<session>/back/ containing:

• summary of lessons learned
• links to interesting materials online

In the repo, the content can be at runs/<year>/<run>/sessions/<session>/front.md etc. (derived from Run.path)

The actual text (and links) on the pages is not that important right now. What's needed:

• Pages at the given URLs that show content of Markdown files
• Navigation arrows – front page links to first lesson and vice versa; last lesson links to back page and vice versa.
• On the course page, the session heading should link to the front page.

[encukou]

@Krejdom, can you take this? If not done by end of Friday I'll need to do it myself.

[Krejdom]

@encukou I'll look at it.

[encukou]

Actually, leaving the feedback in Gist is fine. Take your time with this one; make it perfect :)