Parameter assigned during content pack creation is not stored

## Expected Behavior
Assigning a parameter gets stored and persisted on the next wizard step and when creating the content pack.

## Current Behavior
After creating and assigning a parameter on the content pack creation wizard, going to the next step in the wizard shows the parameter as unused. Going back to the previous step shows mixed information, the parameter list shows it as used, but the entity list does not. Creating the content pack works, but the parameter is not assigned.


There are no errors in the browser console at least while running on production.

## Steps to Reproduce (for bugs)
1. Open create content pack wizard and fill out the first screen
2. Create a new parameter
3. Assign the parameter to one of the entities
4. Go to the next step and see that the parameter is marked as unused
5. Go back and see mixed information

## Your Environment
* Graylog Version: Graylog 3.0.0-beta.4-SNAPSHOT (30b724f)
* Browser version: Firefox developer edition (66.0b1)