v0.4.0

• add notification framework, enable push notifications (#365)
• Password reset button for managers (#335)
• add event notification view (#346)
• add redis caching (#364)
• add conflicting_shifts signup checker (#361)
• refactor permission management in forms (#354)
• enable PWA (#353)
• Guest Participations (#352)
• event slug in url (#351)
• Plugin Infrastructure (#289)
• display shift status in pdf export (#345)
• display qualifications in userlist and disposition view (#343)
• use rich_text for event description (#338)
• switch form handling to django-crispy-forms (#334)
• several small bugfixes