Refactor Step 3 trial workflow

Problem to solve

Step 3 of trial form allows user to create a new group and apply trial to it.

Currently, the backend api does both actions in one api. The idea is to isolate functionality of the apis.

If a user opts to apply trial to a new group, the workflow can be refactored to:

1. User enters name of new group
2. Invoke existing api to create new group (https://docs.gitlab.com/ee/api/groups.html#new-group)
3. If it fails - redirect back to (1) with errors
4. If succeeds, call api to apply trial /-/trials/apply

Remove group creation code from apply api in trials