Add header box with affected teachers and groups in substitution views

• On/off switchable via constance
• Included in substitutions.html and substitutions_print.html
• Add extra QuerySet functions (affected_lessons, affected_groups, affected_teachers)

Advances #61 (closed)

changed milestone to %2.0a2

added part::frontend type::feature labels

merged

mentioned in commit fbc203aa

Very clean and well-structured implementation of the additional filters!