Prepare django upgrade: Remove use of ugettext

AKModel/forms.py

 from bootstrap_datepicker_plus import DateTimePickerInput
 from django import forms
 from django.forms.utils import ErrorList
-from django.utils.translation import ugettext_lazy as _
+from django.utils.translation import gettext_lazy as _
 
 from AKModel.models import Event, AKCategory, AKRequirement