Materials should link to runs and warn about being just materials

[hroncok]

When I see a page in materials, such as http://naucse.python.cz/course/pyladies/ or http://naucse.python.cz/course/pyladies/sessions/install/ I'd like to see a box that says:

This page is here for self study, if you are visiting a course with teachers, use that:

• ... list of links of this content in runs

Problem: Materials usually don't have homework links. It already happened that somebody from the PyLadies Prague NTK run was looking at http://naucse.python.cz/course/pyladies/sessions/def-str/ and was lacking the HW link.