Generalise notifications and implement SMS notifications

This MR implements some generalisations for the notification system.

1. Use more generic naming in model
2. Create utility library ofr sending notifications
3. Implement SMS channel (Twilio) in addition to e-mail channel
4. Make channels configurable in admin backend

Special attention has been paid to:

• Allow easy addition of channels
• Ensure only configured channels can be selected
• Ensure channels that have been selected, but have becom unavailable, are not tried

>>> from aleksis.core.models import Person, Notification
>>> nik = Person.objects.first()
>>> noti = Notification.objects.create(title="Tag des Raclettes", description="Bitte überbacken Sie Ihre Vertretungsaufgaben umgehend mit Käse 🧀!", link="http://www.raclette.de/shop/", recipient=nik, sender="Feiertage-App")

Closes #165 (closed)