rename class to aktuell in generate_pdf, exclude flags b (Verfügungsstunde) and F (Lehrerfrei) in get_substituitions_by_date

Problem: If the start lesson equal to the end lesson, there was no lesson shown

- Arrange print icon right
- Add line break for header information in LaTeX
- Show one-hour absences with only one number (e.g. 1.) instead of two (e. g. 1.-1.)
- Add dots to lesson numbers in absences

Update Templates | Update views | update SubRow-class | update generateSubtable-function | update events | make everything work | but it doesn't work. Perhaps @hansegucker can help me

- Skip saturday and sunday in Smart Plan (issue #124)
- Remove link "Alle Pläne" (issue #114)
- Refactoring (remove prints)

Navigation arrows, comments, better presentation of week days, add navigation items of smart plan to print mode, remove some margins, remove django material imports in header, add media dir to .gitignore