Create an enum class for bell types

Description

Bell notifications are named by an type and an ID, for example fairteiler-12345. The types are spread all over the code and could be collected in a class with constants, e.g. in /src/Modules/Core/DBConstants. A list of types can be found in #732. After that all instances need to be replaced with the constants.

Goal

Make the code cleaner and avoid typos