Migrate from constance to dynamic-preferences

Rationale: django-dynamic-preferences allows defining global preferences and "instance preferences", i.e. user preferences, the same way. It is also way more sophisticated than constance and provides more possibilities for preference fields as well as rendering options.

Closes #162 (closed), advances #214 (closed) and others.

aleksis/core/templates/core/edit_school.html

-{# -*- engine:django -*- #}
-
-{% extends "core/base.html" %}
-
-{% load material_form i18n %}
-
-
-{% block browser_title %}{% blocktrans %}Edit school{% endblocktrans %}{% endblock %}
-{% block page_title %}{% blocktrans %}Edit school{% endblocktrans %}{% endblock %}
-
-
-{% block content %}
-  <form method="post" enctype="multipart/form-data">
-    {% csrf_token %}
-    {% form form=edit_school_form %}{% endform %}
-    {% include "core/save_button.html" %}
-  </form>
-{% endblock %}