Site improvements

Notes from the sprint:

• Review breadcrumbs. Ideally they should correspond to the URL structure, but specific exceptions are fine. Test this.
• Review <title> tags. They should start with something specific.
• Review the URL structure. Deleting stuff after any / should take me to a reasonable page.
  • Do year-based pagination for runs
• Redesign homepage – use big buttons: "chodím na kurz" / "jsem samouk"
  • Remove the links from the site-wide navbar
• Redesign the list of self-taught courses
• Add links to individual "other topics" to the list of self-taught courses
• Add arrows to non-course lesson pages bases on canonical courses they appear in
• Add possibility to hide solutions
• Remove execution counts from Notebook renders
• Add session-specific times and places
• Add session numbers to titles, breadcrumbs, session pages
• Session page: don't show header if there are no materials
• Add “Add to calendar” button to calendar pages
• Add lesson summaries
  • Aggregate summaries on session backpages
  • Add link to backpages to material lists (if there are any summaries)
• Skip session backpage in ←/→ navigation if there's nothing there
• A Session's → link should go to the first lesson, if there is one
• Add infoboxes to session pages

No files, entry points, or tests are named. Begin by splitting the checklist into a single site area, then locate the relevant routes, templates, and tests before choosing one item; done should mean that item is implemented and its behavior is covered by a focused test.