Remove django_dynamic_preferences

[jeriox]

As the library is not maintained very well, it already caused us a lot of headaches and we needed some customization, we want to get rid of it altogether.

This includes finding alternatives for all of our current usages:

• [x] User notifications settings -> moved to the user profile as of #1143
• [ ] event type settings -> could become proper fields on the model or a JSON field for all settings on the model
• [ ] global settings -> probably a new InstanceSetting model that acts as a key-value store
• [ ] internal settings (last run of cronjob etc)

In a second step, we would need to remove the data migrations created for the usages above after a release to be able to remove the dependency.