Use or create external library for template mailer

Along the lines that we want to keep only AlekSIS-specific code in AlekSIS, and contribute to or create libraries for everything that is a useful addition to the Django ecosystem in general, we should check whether a good Django app providing the functionality in mailer.py exists or, if not, create one.

We already had a discussion about how to do good mails, in regard to text/html vs. text/plain, how to include icons, etc. that should be taken into account.