Show announcements in timetable views

Blocked by AlekSIS!184 (merged)

Closes #63 (closed)

aleksis/apps/chronos/forms.py

 from django import forms
 from django_select2.forms import ModelSelect2MultipleWidget
+from django.utils.translation import gettext_lazy as _
+from material import Fieldset
 
 from .models import LessonSubstitution
+from aleksis.core.forms import AnnouncementForm
 
 
 class LessonSubstitutionForm(forms.ModelForm):
@@ -17,3 +20,6 @@ class LessonSubstitutionForm(forms.ModelForm):
                 ]
             )
         }
+
+
+AnnouncementForm.add_node_to_layout(Fieldset(_("Options for timetables"), "show_in_timetables"))