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/preferences.py

@@ -10,7 +10,7 @@ from dynamic_preferences.users.registries import user_preferences_registry
 
 
 general = Section("general")
-theme = Section(_("theme")
+theme = Section("theme")
 mail = Section("mail")
 notification = Section("notification")
 footer = Section("footer")