Allow to configure event rules without a source integration

[nilmerg]

The event rule configuration must allow to configure an object filter for sources of type "generic" even without a hook providing the currently needed forms and filter serialization.

Goals:

• Creating a new rule for "generic" sources does not result in an error complaining about a missing or disabled module
• Clicking on the button to configure an object filter opens a modal where
  • a filter editor is shown without completion and validation capability
• Saving such a filter editor still serializes the filter as a query string